Delegates hold most of the power in your local, state and national party. The delegates vote on the bylaws, the platform as well as the manner in which the party will operate.
The most powerful tool the delegates have is they can actually decide what candidate gets to run on their parties ballot. Yes if they don't like a candidate running they can actually exclude them from beong able to run on the ballot in their party even if they have been in the party for 100 years.
Delegates can require certian provisions as to the positions and policy the candidate must uphold before they run on their ballot.
The delegates have almost complete control of their local party.
Now do you understand why it is so important to become a delegate?