doing ranked in the preseason will help determine your MMR, which is the hidden number riot uses to determine how good you are, basically. your MMR stays with you and increases as you improve. when you finish the 10 matches required to determine your rank, your MMR is factored in. preseason is a good time to improve it if you can win a lot.
in the new season, your rank will be reset. you will go back to being unranked and have to do your provisionals (10 placement matches) over again, but your MMR will stay with you.
a lot of people recommend doing ranked in the preseason just because it doesn't count for anything and it can increase your MMR but people also don't take it as seriously because they know ranked will be reset soon, so it's kind of a shitshow