Originally Posted by xenole

Tier points usually, but price could be a factor.

LHR-SFO in CW may cost £2500.
SOF-LHR-SFO in CE/CW may cost £1200.

OK, it takes longer, and you need to get to say Sofia, Amsterdam, Zurich, Dublin or wherever, and there's always some risk in missing a flight etc., but when you could be paying half the price (and maybe hit gold in one go), it could be worth it to some.

A simpler reason is a final push for status. If you need 80TP to hit gold, your tier point year ends in a few days, doing something like MAN-LHR-MAN in the space of a few hours is easy. Not everyone can do a longer trip, but a free morning works for them.