Rehabs are just a normal part of the rental property life-cycle. You shouldn't have to do a rehab after every tenant turnover, but there will be times when a tenant vacates and you have quite a bit of work to do.
This can range from the standard new carpet and paint to full on bathroom or kitchen upgrades. Rehabs can be pretty pricey, especially if it's been several years since you've done any serious amount of work on the property.
It's also important to note that rehab is above and beyond the normal work you have to do during a standard tenant turnover that may just consist of some cleaning and touch up paint. If you just had a tenant move out after only living there for a year, chances are a lot of the repairs and cost for you to fix stuff that they damaged can be charged to their security deposit. A rehab is usually required after several tenant turnovers or after a long term tenant. After years of wear and tear, you are eventually going to have to do more than swap out the carpet and paint the walls. It will also be required if you get someone who trashes your home.
What needs to be done during a rehab
Every decision you make regarding what appliances, carpet quality, paint, bathroom upgrades etc should be made with the intention of earning a return.