It's common to think of rest days as time away from progress, when in practice they're part of how progress actually happens. Muscles and other tissues adapt to exercise during the recovery period that follows a workout, not during the workout itself, which makes planned rest a functional part of any routine rather than an optional add-on that gets skipped when a schedule feels tight.
Exercise creates a temporary level of fatigue and stress on the body, and rest gives the body time to recover from that stress before the next session. Skipping recovery consistently, by training the same muscles hard day after day without a break, tends to lead to declining performance and a higher chance of feeling run down over time, which can undermine the very consistency a routine is meant to build.
Rest doesn't have to mean total inactivity. A full rest day with no structured exercise is one option, but a lighter active recovery day, involving a relaxed walk or gentle stretching, can also give the body a break from harder training while still keeping a routine feeling active. Which option makes more sense on a given week often comes down to how demanding recent sessions have been and how the body is generally feeling.
Planning rest days in advance, rather than only taking one when feeling exhausted, tends to work better for long-term consistency. A routine that includes one or two rest or active recovery days each week gives the body a predictable recovery window and can make it easier to show up with full effort on the days that are meant for harder training. It also removes the guesswork of deciding, mid-week, whether a given day should be a workout day or a rest day.
Treating rest as a scheduled part of a fitness routine, rather than a sign of falling behind, tends to support steadier long-term progress. A body that's allowed to recover consistently is generally better able to keep performing and adapting to whatever training comes next, which is ultimately what most fitness routines are trying to achieve in the first place.