8 month old still cries at tummy time so I rarely bother. However I do blame lack of tummy time for the small Buddha i have who is happy to sit ALL day with no attempts at rolling/crawling/bum shuffling.
My dd HATED tummy time now she's 15 months old and loves lying on her tummy reading her books or playing with some of her teddies. Keep trying it but don't stress out about it. So not worth it (where's my time machine so I can go back and tell myself that).
My 9 wo hates tummy time, she screams blue murder, however, she goes swimming once a week and spends loads of time sat up right on us or on her front on our chests. She is starting to be able to engage her muscles and we can feel her taking a lot of her own weight when she's sitting (iyswim) so I'm not going to over worry myself about it
Its good for coordination, upper body control etc. Most babies hate it to start with though. Try putting her on tummy with a rolled up towel underneath her arms, just for 30 secs or so. Don't sweat it if she doesn't tolerate it. Any position where she's not on her back is fine. On your chest, sat upright on your lap etc.