The reason changing pads have straps is to secure the baby to the changing pad and changing table to prevent the baby or toddler from moving and falling off the changing table.
When you change your babies diaper using a changing pad on top of a dresser or something else high up then you should use the changing pad straps to secure the baby.
But if you watch the baby closely which you should anyway then the baby should not fall off and you may not need to use the straps.
Whenever you change a babies diaper on a changing pad or changing table then you need to keep a hand on the baby when you look away to grab a new diaper.
If you're changing the babies diaper on the changing pad on a bed or couch or floor then you don't need to the straps.