If you have a lot of birthmarks/nevi/moles, it is advisable to have them checked (mapped) by a dermatologist 1-2 times a year, as the risk of developing melanoma is higher than for those with few moles.
Mapping birthmarks/nevi allows us to track possible changes over time and therefore there is no need to cut them all off.
A skin tumour will be removed if the spot, nodule or other formation shows signs of malignancy or if the total amount of changes is sufficient to remove it. Growths are also removed for non-medical reasons, at the patient's request.