Tor: When should I use a bridge?

Short answer

Use a bridge when Tor Browser cannot connect directly because of network blocking, or when a direct connection to a public Tor relay would draw unwanted attention. In Tor Browser, open Connection settings and choose or enter a bridge. A bridge may slow the connection, so it is not a required setting for every user on an unrestricted network.

Sources

Tor Project: Using bridges, Tor Project: Tor Browser best practices.