Lamination is primarily to slow down fading from UV light. Mainly for outdoors, but also good for longer term window graphics etc. A secondary benefit is to protect the sticker from abrasion, so if it is going to be in a high wear area then it is an idea.
Lamination is standard for vehicle wraps and outdoor signs, and is usually included in the price. On smaller/lower value items you should probably have lamination as a chargeable extra, giving the customer the choice (it also gives yo a price advantage when quoting).
If you laminate unnecessarily then you can end up with a backlog of items 'gassing out'.