There are many ways to edge a bed or lawn, edging keeps mulch in and grass out.
Interlocking pavers make a permanent border with round curves, and eliminates weed whipping by allowing the lawn mower to easily run its wheels on the pavers. The best edging method in my opinion.
Edging done with a bed edger looks natural and is fairly inexpensive but needs to be done every time you re-mulch.
Plastic edging is inexpensive but tends to heave from the freeze, thaw cycle.
Metal edging costs more than plastic but is more durable