What's the purpose of 5.5mm concrete drill bits?
Simply, they're for 5.5mm plugs.

Red screw plugs are normally either 5.5 or 6mm, it's not the case that the manufacturer is advising a smaller drill size to account for poor quality. 
5.5mm tend to be a little cheaper though as they're normally made by companies that sell only to tradespeople in boxes of 1000 without spending on packaging or marketing.  
Fwiw, 5.5mm plugs in a 6mm hole will do quite a good job...which is good because 5.5mm bits are relatively rare.

It's a bit of a discrepancy in (uk) standards...in that there really isn't a standard. The UK isn't very good at adopting standards in the construction trade.

Want some blue plugs? The 8mm or 10mm ones? 
Want 7mm plugs? Brown or grey?
Want some 40mm waste pipe? Sure, how big is it? 41mm, we wouldn't want to be the same as the other guys. 
Want to wire a house? Can I interest you in a ring main?

Incidentally, 7mm are pretty common but not nearly as popular as 6 or 8mm.