Skateboard risers can be useful or they can be an extra part. It depends on what you want.
A riser is a block or pad that fits in between your baseplate and your deck. They are between 3/8" and 1/4" thick usually. I am sure you can double up or find really thick ones from back in the day.
Be sure to get skateboard hardware that is long enough. The risers may make some hardware too short.
Risers can be used for:
I used to use rubber risers as shock absorbers. I put the thin rubber riser on and hoped that it would make breaking my board less likely. I think it worked but there was no way to really tell.
The theory is that if you increase the distance between the tail and the ground that you will have longer to accelerate the board when popping a trick. This higher speed should give you more pop in your ollie.
I would love to test this in a lab one day but for now it is just a theory.
Risers can add extra colour and style to your board. If you are into customizing then you can get risers of many different shapes and colours. Like these:
If your wheels are larger then 60mm wheel bite may become a problem. When you are turning the wheels might rub so hard that they actually stop the board from rolling. This is an even bigger problem if you have wheels that are wide and square shaped in profile.
You can use risers to increase the distance between the wheels and the deck. Therefore reducing the change that they will bite and pitch you to the ground.
If you like going fast and having big wheels then risers are for you. If not then they may just be extra parts that you may not need. I ride my street board without.