This isn't the type of market you pop down to and buy your weekly shop, this is wholesale. It's largely where restaurants will go to to buy their meat for the day/week. Smithfields opens at midnight and closes at 7am. You can walk past at mid-day and it's just a fairly run-down part of london in the middle of other much nicer parts of London. This is nothing to do with walkable cities.

I'm aware of its purpose.

The connection here is that restaurants are part of the appeal of walkable cities and now their life will be made harder. Net effect will of course be an increase in prices.

You need places like these in a city because they shoulder a huge burden, namely logistics, which are particularly hard in densely populated areas.