Hi. Good question.
Couple of reasons.
Firstly for extra strength.
the main tubes are Reynolds 853. It's an air hardening tubeset and brazing doesn't get the tube ends hot enough to get the process going. Brazing is in the 800-900 deg range and TIG is 1500 deg range. the tubing needs to get to 1400 deg minimum to get the extra strength.
Secondly, the fillet will help dissipate stresses further along the joint and prevent cracks forming along the scales of the TIG welds- the most common starting point for a joint failure. The small scales of the welds can act as a stress raiser. My background is automotive- mainly race cars and this is where i've seen problems in the past. I suppose its different from what most frame builders think but i'm just applying 20 years of experience from other sectors to make sure my frames are strong enough.
Finally, i love the oldschool looks of a fillet brazed joint. Yes there is a slight weight penalty, but I'm not a weenie. I want something that will ride well for years to come. i won't compromise this just for the sake of a few grams here and there.
Incidentally, true temper (who i use for my seat stays) did lots of destruction testing on joints joined by braze, tig and a combination of both and the tig joints always failed first, and the the joint always broke. the brazed and braze over Tig joints lasted longer and the tube distorted around the joints but didn't snap- in other words, if this type of joint fails, you still get to ride home and keep your teeth!
cheers
Si