Ease of access and ability to divide utility bill for multiple units

Needing access to the subpanel is oftentimes necessary when you are renting out part of your home. If the main panel is in an area that is off limits to the tenant and there is a breaker that trips on one of the tenant's circuits then they are forced to wait for the landlord to reset the breaker.

A subpanel would be common in a duplex or a large house that has been converted to apartments. This would also make it easier for the electricity bill to be broken up between various parts of the house in cases where a tenant pays their portion of the electric bill that is seperate from the rest of the house.

Limitations due to size of panel and or number of amps

Your current main panel (which every house has) may only be rated for a certain number of amps or there may no longer be any available circuit breaker slots.

A subpanel is significantly less expensive than adding wiring to an existing panel

When a subpanel is connected to the main service panel box it only needs the hot connections and the neutral wire. Whereas any branch wiring would need a hot wire, a neutral and a ground running from the main panel to EACH circuit. Therefore it can be less costly to install a subpanel that has circuits with shorter runs than to run circuits from the main panel that travel much further throughout the house.