Prequalification is a basic review of your finances to determine if you would qualify for a home loan and if so an estimate of the size you would qualify for.
The benefits of getting prequalified for a home loan is that you will know what your credit score is and what you can realistically afford in home loan repayments every month. Another added bonus of being prequalified is that sellers will see you as a serious buyer, which could give you the edge in a competitive market and you won't be wasting your time looking at properties outside of your affordability bracket.
It is important to remember that it is not a guarantee of a home loan, but it’s a very good indicator of what size home loan the bank will grant you.