Blowjobs often inhabit a grey
Blowjobs often inhabit a grey area between foreplay and orgasm. By default, I tend to think of them as foreplay, and hold back a little in the assumption that my beloved is soon going to want a good slamming.
But there's something very powerful about the notion of permission. Having a woman look up and say "I want you to come" or "come in my mouth baby" can flip the switch on the whole encounter. All of a sudden it's like "Woo hoo let's go for it!" ...And on the other hand it's helpful to know if you're just warming me up.
By the way, having an orgasm from oral sex is less common than having intercourse, and way less common than it seems to be in porno movies (speaking from my own experience here). This means that if you give great head it will be remembered fondly and for a long time. I got a blowjob in February of 1990 that I'm still recovering from. Let alone a couple of more recent standouts.