3+ reasons for buying via OTA
1) they may extend 24hour cancellation (or even 24+hour/or over weekend) for flights that may not qualify under DOT airline rule
2) eg orbitz 2% reward for booking flights, plus any cashback sites on top of it
3) want to book a ticket that UA.com wont quote
~4) possibility of getting BULK ticket and BULK earnings, very YMMV
2+ reasons for not buying OTA
1) may have to change/cancel - OTAs may charge a fee on top of UA fee
2) any issue with ticket (eg schedule change, or causing misconnect), you may have to jump through hoops and OTA/airline will point ticket at each other
~3) OTA may issue BULK fares which you dont want. YMMV
if im travelling for leisure on a supercheap fare, i would go OTA to get the extra %