Old main panels have limitations on available space to add circuits or they have fuses (some insurance carriers will not insure a property that has fuses).
Some old panels have had circuits added and have maxed out (most old panels are maxed out when the original construction was completed). Many old panels are of inferior quality and simply stop working and need to be replaced.
An electric service upgrade is often recommended or required to increase the capacity of the existing electrical service. With all the new appliances and technical devices we now have in our homes, we find that older homes just don’t have sufficient power available to handle the increased demand. Therefore, a “service change” is required. This may include not only upgrading your electric panel, but upgrading your meter socket, the wire between the meter and panel, the wire between the utility and the meter and the grounding system as well. A panel upgrade or replacement is usually required to alleviate a problem with the existing panel.