Derailleur hangers have the critical job of holding your rear derailleur perfectly parallel to the centre plane of your frame (if you cut your frame perfectly in half with a laser that would be the central plane of your frame). They can get bent from a crash or if you lay your bike down derailleur side down and at best you get poor shifting performance. At worst, when you shift into the biggest cog on your cassette the derailleur gets caught in the wheel spokes and is ripped off while you get thrown from your bike. You should check it regularly and if it’s bent have a mechanic straighten it for you. If you need to order a replacement, remember they are specific to the brand and specific model of your bike. Check out the wheels manufacturing website for help choosing a replacement.