If you are from the EU, then you should definitely do it. For customers outside of the EU it doesn't make sense, this does not apply to you.
IOSS makes everything easier for you, and potentially lowers the risk of customs confiscation, since they see it's already paid for.
If you don't do it, you have to take care of the customs payment yourself. This can be difficult, as with many shipping services you have to pay at your door, meaning you have to receive it in person.