Apologies for the somewhat vague title and incoming speculation but why do you think Oda so thoroughly introduced Big Mom and her crew only to have the Strawhats not fight her yet? Admittedly I may be am jumping the gun a bit, who knows what may happen in the coming chapters, but currently it seems the Strawhats will run away, meet up with the others and have a huge, long epic fight against Kaido/Wano.

So for some reason the idea that they are going to fight Big Mom for real after what will almost certainly be a ridiculously long arc against Kaido strikes me as unlikely. Also going up against Big Mom after introducing so much of her after fighting another Yonko in Kaido would seem like retreading known ground.

I suspect instead Big Mom will be taken out by someone else in some chapters shown immediately after the Wano arc is over. My top suspects would naturally be Blackbeard or another pirate alliance, although a Marine/Shichibukai team up taking her out would be interesting.

With that (total speculation) in mind I suspect the current arc happened for three main reasons:

1. Give some backstory to Sanji. Not really the point of my post but I feel I should acknowledge that as a thing that obviously happened.

2. Give us a chance to get to know Big Mom's crew before she's taken out by someone else.

3. Perhaps most importantly, give us, the readers, a taste of what it's like to go up against a Yonko. With the incoming fight against Kaido it's good to setup a bit just what we're in for. The Zou arc also illustrates this to a degree, introducing us to Jack and the sort of strength a top Yonko subordinate has. Bege's presence is important for this reason too, a top rookie/worst generation type was treated as mid-tier by a Yonko.

EDIT 4. As /u/JimBae points out below they got the poneglyph info as well, which will be important in the future. Which to me only reinforces my theory, as with that info there's virtually no reason plot-wise for the Straw-hats to return to Big Mom and beat her up. /EDIT

Other points, such as Luffy struggling against a top commander while Big Mom's strength is obviously well above that. In fact Big Mom being on her way to confront them directly now is another good way to get a taste of what a Yonko's personal strength is like when they finally encounter another Yonko in Kaido.

TL;DR - My speculation is that the WCI arc is about setting up what a Yonko's strength is and getting to know Big Mom's crew before they are defeated by someone else, possibly off-screen.