Other than on a sunny day, an unheated hoop house will be the same temperature as the outside temp. It may save them from drying out in the wind, but the temp won't be much different.

It can certainly help protect them from frost damage, especially if you have a mild winter.
Also depending on which annuals you are trying to overwinter some are more cold tolerant than others.

I tried it last year for the first time and it worked. I made mine with cinder blocks and PVC conduit covered with plastic sheeting. It was very easy to put together and dissemble. I also had a 100 watt bulb in a trouble light that I turned on the coldest of days.

If you have it in full sun you will have to open it up on sunny warm days which are more often than you would think.

I have mine set up under an aluminum patio cover and it only gets late afternoon sun. I have one side open most of the early winter and only close it when there is a chance for frost or really cold weather.

All my plants were in pots. I was able to overwinter dragonwing begonia and several tropical plants including a pineapple and geraniums. I also overwintered wave, supertunia, tidalwave and shockwave petunias with no cover other than moving them under the patio cover on really cold days I fogot to move them on somedays and they survived being snowed on.

It is amazing especially with the dragonwing begonias to start off early spring with plants that are huge and blooming right of the bat.

I plan to make mine much large this winter with many more plants.

Trim the annuals back prior to putting them in the hoop house.

Make sure that if they are in pots that they have good drainage and good draining soil because during the winter rainy months many plants that may have survived the cold drown from poor drainage.

Good luck