In some regions, if you reach top 500 MMR and want to queue at off-peak hours without being queued with diamonds (or sometimes even plat players) its easier to just have a smurf account that you can play on. Otherwise the game puts you up against a team of mid-high masters players and the average SR of your matches at these off times will often be 6-700 SR less than your rating and you can end up just feeding your SR away. These games often just feel shitty for everyone involved.

Also handy for practicing other heroes that you might not have at top 500 caliber.

For anyone wondering why someone might have 3 accounts and not just 2: once your smurf gets to top500 the problem just repeats if, for example, you want to practice another hero (and do so anonymously if people have identified your smurf). Even if you were to let one account decay its SR, your hidden MMR is still what matches you so you need a fresh account or to tank your MMR which would essentially require throwing games.