to authorize the agent to keep a set of key (usually)

so that they are authorized to let prospective tenants to view the unit without needing to get the key from you each time there is a prospect.

they definitely cannot decide on the tenant, rental rate, or T&C of the tenancy on your behalf. The decision maker would still be you.

as for cons..agent got your key lor. u dont know what they do inside.

anyway my two cents.. usually hardworking / hungry agents wouldn't have time to mess about in your unit and would rather use their time to close deals and get more $$