You need to trim your piggies nails so that:
They don't get too long and get caught whilst she is running around / or being held- risking her breaking a toe or ripping a nail out
You don't want them to get too long so the nail curls round and grows into the soft bed of her paw (can happen mostly in the front paw) which will be incredibly painful and will become infected
Your piggy will feel more comfortable running around with neatly trimmed nails and less likely to injure herself when scratching etc
Your piggy may sometimes grow 'spurs' on the side of her paws - these can be painlessly clipped off when you trim your piggy's nails to prevent them catching and tearing - if you are not 100% certain that what you think is a spur [sort of like hardened nail growth but not from a toe, from the side of her paw which can also curl under the foot if not clipped] is a spur then don't attempt to cut it - get it checked by your vet so you don't accidently injure your pig.