Here's a question: When Wi-Fi goes out, why does restarting it fix the problem?
Think of restarting your router, or router-modem combo, as a way to refresh the device.
Routers are essentially tiny computers with limited processing power and memory. The more you ask it to do, the harder it has to work.
If you leave your computer running for days on end, your router can slow down or hang up if you don't give it a break. Restarting it clears its memory and resets tasks that might've stalled.
One of its most important jobs is handing out unique IP addresses for each device that connects to it. If you have a lot of devices requesting addresses or devices frequently jumping on and off the network, the router sometimes can't keep up.
The modem can also lose its connection to your internet service provider, especially with DSL. In that case, restarting is really your easiest option to reconnect.