Why exchange c-pawn and d-pawn in Sicilan

Random question from a predominantly French player looking to expand his repertoire:

After 1.e4 c5 2. Nf3 e6 (or d6 or Nc6 or whatever you want it to be) 3. d4, why not play something else (Nc6 or e6 or whatever you didn't play before) and let White make the trade? Why does it seem like taking on d4 is more or less "forced"? I don't know much about the Sicilian, but to my amateur eyes, I would prefer to maintain the pawn tension and maybe develop some other piece.

Because it's possible for white to play d5, with a benoni structure.

johncorean, I've heard of people sometimes delaying the capture, after say 1 e4 c5 2 Nf3 Nc6 3 d4 Nf6 which for some reason avoids lines with 3 d4 cxd4 4 Qxd4 (don't ask me why, it's not the point). So I'm not sure if white can, right after ...Nf6, favorably push his pawns, but in general although you're keeping the pawn tension, it's white who retains all the options by you doing that, not black. Black's only choice is to take at some point until white does something while white could maybe play dxc5 or d5 favorably at some point.

It's not that he can necessarily do this right away, it's that you're giving your opponent more options that could be good later.

Thanks for the quick responses. As a French player, I'm getting a little bored with the Advance Variation, which I seem to be facing 90% of the time, so I'm thinking of switching up.

The reason I was asking about 3... cxd4 is that I'm thinking about experimenting with the following line: 1. e4 c5 2. Nf3 e6 3. d4 Nf6. 

I haven't played it before so I haven't really looked at possible variations. But my current thought is the most natural White response would be either 4. Nc3 or 4. e5. I would probably play 4. Nc3 Nc6 5. d5 exd5 6. exd5 Nb4, or 4. e5 Ne4 5. Bd3 d5. 

Again, none of this has been fleshed out, but I was wondering if anyone would be interested on commenting? I'd especially like to hear from Sicilian players on the White side.

Ha! That's exactly the reason I stopped playing the french so much. Sheer boredom from seeing the same structure time and time again. I found the e6 sicilians to be very much up my alley as a french player. There are great similarities of structure and plans.

Oops! Ignore 4. Nc3 Nc6 5. d5 exd5 6. exd5 Nb4, that doesn't seem good at all after 7.a3. Like I said, haven't really thought it through.

I would have never thought it's the advance that gets french players to not play!

It's not a theoretical problem, it's a boredom problem. Seeing one and the same position in 90% of your games with black gets slightly tedious after awhile.

I have never understood why people are so eager to make that pawn trade. They do it against the English all the time to, playing some Anglo-Scandinavian that trades their d-pawn for my c-pawn, moves the queen out, and gives me an extra tempi when my knight chases her away.

I always love that opening, and have no idea why black plays it.

Free development for their pieces, that's the idea.

That's kind of what I don't like .

Well, at last I've read all the posts (I'm not so good at English actually).
Thank you for your many many answers, I guess I can't get some of what you said unless I pile up more experience.
But I got some ideas with exchange cxd4.
I try more games. Thank you very much.