Indiana might have trouble resigning Turner so it might be wise to get some assets for him before he potentially walks for nothing. This trade gets them a secondary ball handler in Reeves as well as an adequate center replacement in the short term.
The Lakers get much needed center help as well as add a couple of depth pieces. While it hurts to give up Reeves, he is a little redundant next to LeBron and Luka, so this trade allows them to fill out their roster better.