I would say simply because it is usually significantly quieter than either T5N or T5S, and has the same food/drink offerings.

That does vary with whatever flights are heading out of T5B of course, earlier this year it did backfire on me quite badly. Taking non-status friends there, as I guested in, I explained we were going to this lounge for that reason only to have the lounge angel suppress a laugh and then explain that one of the longhauls (SJC from memory) had been badly delayed and the place was rammed full of irate passengers! Luckily they were called within about 10 mins of our arrival and it was restored to a (relative) oasis of calm.