A mentor helps you because they like you, your cofounder(s), you + your cofounder(s), or you + your cofounder(s) + your potential.
A great mentor can be absolutely invaluable as a functional add-on for your team. Think of them as an experience amplifier. They will also not qualify advice they give because they're not trying to sell it to you, which is what a startup consultant does.
Advisors often receive small equity grants in exchange for 'advice' or other material help for your startup. They are usually not angel investors or they missed your angel round.
Often they may do a variety of functional/operational things like help you find/source talent, come up with a basic engineering or marketing plan, etc. However, they usually want to be 'known' as an advisor to you, so proceed with caution. Mentors will help you because they find you deserving of help, not because they want to see their name on your team page or get some options.