Most of my match and special purpose rifles are built on Remington 700 actions and use aftermarket barrels from Hart, Shilen, Bartlein and Kreiger. The other few are AR-15 platforms with Kreiger barrels.
The main reason is enhanced precision, since most factory rifles aren't competitive as they come. The only two precision rifles I use that I am still using the factory barrels on are a Remington 700 LTR and a 5R, due to the factory barrels delivering fine precision as is.
Just replacing the barrel isn't usually enough to bet on, and so when doing a barrel replacement, the actions are squared/trued/lapped as is appropriate to help deliver sufficient precision.
The secondary reason for replacing the factory barrel is to use a cartridge that is not available as a factory offering. My 6.5-300 WWH (1k BR) is one example, others include .35 Whelen (hunting) and 6.5-284 (1k F-Class).
I have also replaced barrels as an expedient way to have a threaded muzzle for suppressors. Having a gun taken apart and threaded will cost more than $150 and one barrel I took off myself to have threaded still cost $165 (Engage Armament). I have replaced several CZ455/457 barrels simply to have threads where there were none.