This will be a quick post inspired by a mistake I often make in my own training.
Warm Ups are important.  Getting tissue warm and ready for exercise is important for maximal performance and injury prevention, especially if that warm up is dynamic and mimics the kinds of movement you are about perform in your exercise routine.
With that said, after you have warmed up and performed a lighter warm up set with a specific exercise for a specific muscle group, GET TO YOUR WORKING WEIGHT. 
For example, I was squatting today. I really do not like (the act of performing) this exercise.  I found myself warming up with 135.  I went to 185 next. Then 235.  Then 285.  Then 335, which is my working weight for my target reps number of 8.  So, I ended up with one set at my max potential weight of 8.  That’s one working set out of 5!  That’s not good. 
To force an adaptation (change) in your performance or physique, you have to work out at your max potential for all sets except for a warmup set (or 2 for bigger movements like deadlifts or squats).
I see people in the gym doing “chest day” where they will perform 3-4 warm up sets for every single exercise, even though they are training the exact same muscle group at a slightly different joint angle.  At the end of the day, a routine like this will give you 5-6 sets that have a potential to change anything, with the rest acting as a combination of warm ups and calorie-burning weight lifting.
The point: Warmup and get to the point.  If an exercise routine calls for 4 sets of 8 reps, that means 4 sets to failure at 8, which is your MAX weight, not a slowing ascending ladder of arbitrary sub-max loads.
Matt Pirtle, MA, CSCS