You probably have heard all about how you need to vary your exercise routine in order to continue getting results. Terms like "muscle confusion" (a made-up term with no backing) have even been thrown out to indicate that for best results, you can't always do the same things. To an extent, this is true. You can't always do the same movements at the same number of sets, at the same number of reps, at the same speed, at the same weight, and expect to improve (we'll get to this soon). The problem is that historically this advice has been interpreted as "do different exercises all the time", and thus people do different exercises every time they work out. There are a few problems with doing that, however.
1. Lack of Practice = Poor Form
If you stop thinking of your workouts as training muscle groups and start thinking of them more as practicing movement patterns, you will improve rapidly and reduce your chance of hurting yourself greatly. To get good at anything, you have to do that thing a LOT. If you constantly mix up your exercises, chances are you aren't going to improve as much in any movement because you don't practice the form enough. To get good at a movement pattern you must do it over, and over, and over until it is absolutely ingrained in you.
Squats are a great example of an exercise that requires a lot of concentration and attention to detail. It's a simple concept, but more goes into a squat than most people realize. If you only do squats every two weeks, you'll have to waste time relearning the form every time only to realize that all those fancy movements you've been trying didn't really translate into squat improvements.
The second problem is that your progress is too hard to dial in when you constantly do different movements. Especially for beginners who already aren't very in tune with their capabilities yet, it can be hard to know how much to increase the weight on a particular exercise if you haven't done it in weeks. By consistently doing a movement, however, you become very aware of your capabilities within that movement and can adjust accordingly as you get stronger. This leads to much faster progress and hugely reduces the chance of injury.
It is also much more motivating to see consistent small improvements on a weekly basis than clinging on to hope that you improved after several weeks of training. In terms of sticking to a program, especially for beginners, this is absolutely critical! You need to know that what you're doing is paying off every single week.
3. Too Much Guesswork
If you don't have a plan, it's not going to go well. Period. We've all been there, though. You show up at the gym with some vague idea of what you're going to do and hope to be inspired while you're there, so you make things up as you go along. Or, you spend tons of time concocting a sophisticated "all-encompassing" fitness routine with 60 exercises per week that you have no chance in hell of sticking to anyway. Either way, it's less than ideal.
The beauty of routinely doing the same exercises is that it takes all of this guesswork and pie-in-the-sky thoughts out of the equation. You know exactly what to do when you show up at the gym and never have to stress about that. As a beginner, even just showing up can be stressful. Don't give yourself extra things to worry about like which exercises to do for every single workout.