Not exactly sure what you're asking.. Do you mean why did it go off when it did? It was either mistimed, or they assumed that with all the chaos nobody would have noticed the few seconds of the explosion before the plane struck the tower, being that the explosion was at the basement of the tower.

Or do you mean why have explosions in the basement in the first place? To weaken the infasturcture of the tower obviously. If you want to take a building like that down, you gotta crack it from the inside, and let it collapse in on itself.

Check out the William Rodriguez videos. Very fascinating stuff.

[edit:  Upon reviewing the thread, I don't see where I got my idea of your claim, so I may have got it from hudson's post -- "Just seconds before you hear the planes hit there is a clear thump from an underground explosion" -- and misattributed it to you.  You and hudson may well, in fact, not believe what I was questioning.  See below.]

You claim there were explosions before the planes hit AND before the towers fell.  (If I somehow misunderstood something, and you only think there was one explosion for each tower (which happened when the planes hit), then oops, my mistake I guess, and never mind the rest.)  Obviously the second explosions were to bring the towers down.  But what is the point of the first explosions?  Why would they need to set off explosions when the planes hit, if the actual demolition charges wouldn't be set off till later?  Why have two separate detonations?  It makes no sense.

[edit:  If you agree with hudson, that there were "underground explosions", I ask you:  how could they possibly cause a collapse of the towers than began up at the levels the planes crashed on?] 

Open and shut case when the FEMA report couldn't even explain the collapse of building #7 due to sporadic fires.  Sounds open and shut case to me.   So let me get this straight.  It was okay for Ken Starr to spend about 6.2 million to investigate whether or not Bill Clinton got his dick sucked (39.2 million if you factor in Starr investigating Clinton's whitewater deal and Lewinsky; Besides Starr, five other independent counsels are currently conducting investigations. Four of those focus on the Clinton Administration. The combined costs of those four inquiries and the Starr probe now comes to 79.3 million).  So we spent 79.3 million investigating the Clintons when the government spent 600k for the first investigation of 9/11.

47.4 million was spent by Lawrence Walsh on the eight year investigation of Reagan Administration officials involved in the Iran-contra affair.  Did we even investigate going to war in Iraq?  So we spent all these millions on investigations yet we couldn't even spend 1 million for the initial FEMA report for the biggest attack on USA soil?

http://articles.cnn.com/1999-04-01/politics/counsel.probe.costs_1_special-counsel-probe-david-m-barrett?_s=PM:ALLPOLITICS

I am just pointing out the the priorities seem to be a little out of focus for the USA.   We seem to care 100x more about whether a President got his dick sucked and lied about it then a President, Vice President, and Defense Secretary spreading lies and misinformation to start a war that USA didn't need to fight.   Let me tell you I was so scared of Iraq before Operation Iraqi Freedom campaign.  I could have sworn Saddam was about to attack us...  That must be the reason why we went into Iraq right?  

saddam over played his hand. he wanted to continue to act the defiant big shot expecting that if something did happen, it would just be a limited bombing campaign. well he was wrong.

Yep, he should have known we were more interested in invading his country and trying to privatize their oil rights.

I've figured it all out, it was the ultimate false flag operation!

Everyone "knows" al Qaeda did it, right? But really it was the Israelis, who in turn were doing it at the request of our own government! ...Or so they thought!!

In reality, it was al Qaeda, masquerading as the shadow government that secretly rules the United States, that fooled the Israelis into doing 9/11 and blaming it on al Qaeda!!!

The perfect plan. And no one even suspected them ... until now.

Actually the FEMA report does explain this.

So, i guess you didn't read any of the reports, and instead just blindly went off of what crazy conspiracy theorists have said.

Why do people latch on to points that have been repeatedly and thoroughly disproven for years, to promote an idea that has no logic or common sense to begin with?

Yes. 

I guess you didn't read the FEMA report then. The investigation the FEMA carried out was useless. Just read their conclusions.

The NIST took the investigation in their hands after FEMA failed to find out a logical explanation for the collapse of WTC 7. And their conclusions are even more laughable. They are a big pool of speculative shit presented as facts. I suggest you read the reports before you talk.