Any good personal trainer will tell you that you shouldn't follow the same exercise routine indefinitely - your muscles will get used to doing the same exercises and so in the long run they will use their effectiveness.
It's worth thinking about your diet in the same way. Overtime your body adapts to your diet but also your nutritional needs change. The diet that suited your body in your twenties won't still be perfect for you in your thirties.
Gradually adapting what you eat overtime to your needs is an important part of staying healthy. But it can also be good to adjust your diet more radically for a couple of weeks to wake it up and activate your fat burning pathways!
A detox is a good way of doing this as is a couple of weeks on a carb restricted diet which can help get your blood sugar levels back into balance or help you kick the sugar habit if your diet has gone off track.
Endless yoyo dieting is very unhealthy and I'm against strict diets as a long-term approach to nutrition, but if you want to try out a new diet for a couple of weeks twice a year it can be a wake up call for your body, bring some new foods or recipes into your diet and do you some good.