Easy does not mean useless
The main goal of an easy run is to accumulate running time with relatively moderate stress. This intensity can help build endurance while leaving more room for recovery.
Save energy for key sessions
If every run becomes hard, fatigue can accumulate quickly. Easier days help separate quality sessions from relaxed endurance days.
How to recognize easy effort
A simple method is to observe breathing and overall feel. Effort should remain controlled; for many runners, speaking in sentences remains possible.
This information is general. Training should be adapted to your experience, recovery and personal situation.