That's just more food/drinks/concessions stands.
As for why they are doing this rather than the original plan of a 25k stadium, I would guess there's a number of factors:
i) As the club have said, they're running closer to capacity that ever before. That means that key areas like the Fan Village in the South/SW corner gets very busy. If they can spread the demand over other areas of the ground, that makes managing the fans getting to their seats easier. It'll also reduce bottlenecks like the one you get between the S & E stands that is always an issue.
ii) Adding this area is a lot quicker than building a whole new stadium! They can potentially use this to get permission to expand the North stand as a medium term measure on the way to a complete new build.
iii) Making the space covered but heated means that they can offer this as an extra onsite venue for after game activities even during the winter. At the moment if it's cold/raining the only real options for people to go are the Honours and Kings bar. Once they're full, people will go elsewhere, taking their money with them. Add in this area & you now have another option.
iv) Why haven't we just bought the Council land & gone for the complete new build? Well, there's been this little thing called a Pandemic that impacted on everyone's pockets. Plans will no doubt have had to be pushed back a few years. But, in the meantime the clubs' growth in attendances means they need to do something to make sure that they keep the whole experience of coming to the Stoop an enjoyable experience.
