Ddewey, There are a few reasons you might wish to stall your new horse. If he has not been out on pasture 24/7 and your pasture is actively growing(depends on where you live) is one good reason to stall parttime. YOu would then need to acclimate him to the rich grass by allowing him to graze for short times increasing up to full time. If your grass is dormant and or your horse is used to being on pasture then you do not need to stall.
I only stall my 29 yr old appy, part time, as he is blind. He'd prefer to be out as a rule.