An uneven finish in your painting, some parts matte, other parts glossy can be very off-putting to the viewer.
A unified finish enhances the colours and is a great way to add both a professional finish to your paintings and add dollars to the sale price.
No one technique for varnishing suits every situation. The texture of the paint surface, the desired finish, speed of completion, etc all affect which technique is best to use.
On a side note, it’s also the number 1 trick to making your acrylic paintings look like oils…