Acoustic guitar pickups will allow you to amplify your sound for performances or practice, so people can actually hear you while you are playing.
A lot of open mic events will have a microphone set up for acoustic guitars, but no acoustic guitar pickups. In this case, if you don't have an electric, it is important you have your own acoustic guitar pickups.
There are many options for acoustic guitar pickups out there, and differing prices. If you look up acoustic guitar pickups at Google, you will see the top sites offer acoustic guitar pickups for about $90, $60, and $50.
There are, of course, different levels of quality to consider (I am a musical person with a very limited budget, so the cheapest ones are listed here, from Amazon).
Acoustic guitar pickups will allow you to amplify your sound without having to be in one place all the time. If you are just standing in front of a microphone, your movement is very restricted.
I myself have bumped into the microphone while performing, which is embarrassing. It shows you are not very experienced on stage.
If you have an acoustic guitar, and you want to perform, you should have your own acoustic guitar pickups.