Why should I give my baby tummy time?

Strengthen chest/arm. Babies who learn to sleep on their back often fuss if you place them on their belly, yet tummy time will help with muscle strength in chest & arms for later as they start to roll over. They can acquire this after learning to roll over but may briefly seem ackward or delayed. I wouldn't lose any sleep over the issue & prefer to avoid battles when baby makes a big fuss every time. They will catch up.
Develop neck muscles. Tummy time is important to help your child develop their neck and shoulder muscles. Now that all babies are sleeping on their back they no longer spend enough time on their tummies.

Related Questions

My son is 8 weeks old, when do babies start crawling? I've been working on it with him during tummy time but he can't pick himself up with his arms.

See below. Full-term born infant achieves lifting head by 2 months, roll-over by 5 months, and crawling by 7-8 months of age.
6-9 months. 6-9 months. He is too young for that. Can turn over at 4 months maybe.
You can't teach it. Babies nerves mature from the top down and middle out. Lifting the head and feeding well are the early milestones they have at birth. They can't get their shoulder to tuck to roll over until 3-4 mo, so why would they do such a complex thing as crawling at 8 wks? Crawling requires complex integration of mirrored movements. Most crawl, pull to stand and walk within a few weeks when they get to 9-12 mo.
Keep up the. Tummy time. At 2 mos. He'll regard his fists & get his head up to regard & follow a red or yellow toy dangled in front of him. At 4 mos., he'll get his head & chest up. By 5 mos. He'll be able to bear weight on his forearms, shifting from one to the other to reach for & attain objects. Cat-crawling is variable if it happens at all. It used to be ~ 6 mos., now ~ 7-8 mos. Since " back to sleep".