You might try to get some information from the previous owners about how they maintained their beautiful flower beds. I suspect it was more than yearly applied mulch. Depending on what you're growing, yearly mulching may or may not be necessary, though it is always helpful.

First... 'need' probably isn't the best term. Will the beds benefit from mulch? The answer is almost always a resounding... yes!

'New' mulch every year is usually not necessary. Adding fresh mulch is done yearly by most homeowners for aesthetics. People like the look of fresh shredded material. Keep in mind though, that there are many materials that can be used as mulch, and each one has it's own strengths, weaknesses, and maintenance needs.

If you decide you want to use a mulch, the first thing is to consider the space, the plantings, and what you want the mulch to accomplish. Then you learn how to use the particular mulch you decide upon.

If you're using just about any kind of wood mulch (and I hope you are) , it will be decomposed from the bottom in fairly short order. If I put 4 inches down, it'll be just about gone in 2 years, so, yes, you need to top it off fairly regularly, but it's not the end of the world if you let it go a little too long; you'll just end up with more weeds.

How thick is the mulch now? A 9 year study by two researchers at Michigan State University, reported in an issue of Fine Gardening magazine early in 2016, showed that a thickness of 4 to 6 inches was best. If the mulch in place now is less than that add more. If the thickness becomes less, due to bacterial activity, next year add more.

Try to maintain a 4 to 6 inch thickness.