You can use it to practice in between rounds when you're not competing. When you solve at solving stations timers, mats and everything will already be there. I suggest watching the wca competitor's guide on youtube. but other than that if you watch competition videos you should get a hang of everything. its not that you arent allowed to bring a mat or a timer, you can but when you are going to do official solves you will use the supplied timers.
for the question about getting a timer or a mat, i suggest getting a timer first as its the most worthy thing you can buy. the mat is just to protect your puzzle from slamming down onto a hard table repeatedly for hundreds of solves at home. You can substitute the mat for a big mousepad if you have those giant ones, or even a medium sized one, as long as it covers a substantial area. You don't necessarily need to get a legit speedstacks g4 timer as well. yuxin sells a rip-off g4 timer, and any other speedstacks-style timer is functionally the same. You could get it to go comp sims or just for fun cause it feels nice.
a link to the yuxin timer :
https://www.ebay.com/c/1277326189
also, for the mat i suggest getting a giant gaming mousepad for home usage so you can game with it as well as cubing. I have a medium-sized one.