..it's a sore point for some (literally) and I've been there. Most of us that train regularly will have enthusiastically over-trained at some point and it doesn't feel great.
Do we really need rest days?
Absolutely - you need time to adapt - it's important. The rest plays its part in making you stronger.
Why can't we just keep going?
Well, you can if you want and some do but the likelihood is that your fitness will simply max out and you'll plateau at that level. You will likely suffer from fatigue. Your body will become accustomed to what you're asking it to do but you'll under perform and you'll be more prone to injury. Your body will start to tell you that it needs to rest and you may well get that message loud and clear during your next workout which, whatever the exercise, is likely to feel something like wading through treacle!
In order to get fitter or improve in sport, the body needs to be exposed to stresses (i.e. training or exercise), once this has happened, the body then needs time to adapt to the stresses and for this there must be a period of recovery.
For those new to exercise, if you just keep going, without any rest, your body will soon start to fatigue and you’ll find it more difficult to complete your next session.
For those who are more experienced exercisers and are maybe training for an event, rest and recovery is also vitally important. So, what's ‘Progressive Overload’?
• Training that is designed progressively to overload the bodily systems and fuel stores.
• If the training stress is insufficient to overload the body’s capabilities, no adaptations will occur.
• If the workload is too great (progressed too quickly/performed too often without adequate rest), then fatigue follows and subsequent performance will be reduced.
• Work alone is not enough to produce the best results; you need time to adapt to training stress.
• To encourage adaptation to training, it is important to plan recovery activities that reduce fatigue.
• The sooner you recover from fatigue, and the fresher you are when you undertake a training session, the better your chance of improving.
If you are serious about getting fitter and stronger and performing better, train hard but also plan in adequate rest periods. Adaptation is your friend