It sounds like they're warming up with 15, 20, and 30, doing one work set with 40, then doing what's called a "back-off" set with 10. They do the warm-ups in order to safely get up to a heavy weight. They do the heavy weight because heavy weight is the most effective. They do the back-off set because they're tired from the heavy weight and they want to get another set in.
Mark Rippetoe, who has what one might call a slight pro-strength/anti-bodybuilding bias, has this to say about back-off sets:
Back-off sets at a lighter weight can only add volume to the workout, and are usually done to focus on a technical problem within that lift that can benefit from some concentrated attention being paid to correcting it. A lighter weight done as a back-off set cannot drive up the weight on the work sets.
...Volume can drive progress for a short while, but the main work must be done in the rep range and with the weight called for by the adaptation you want to drive. If you want strength, 3s-5s are what you have to do and lighter 12s will not make that adaptation occur, whether done as a back off or as work sets. If you begin to rely on your back-off sets as the primary stimulus, you will stop progressing.
Personally, I've found back-off sets quite useful in two scenarios:
- Working on max singles and doubles as well as the 3-5 rep range. I'll work up to several sets of 1 or 2 reps each, but if I find my form degrading before I get enough total work done for that exercise (say, only 2 reps in the deadlift, or 6 or 8 total reps in the squat), I'll sometimes add a lighter 3- to 5-rep back-off set that's still challenging.
- Overwarm singles. I'll work up to one really heavy rep right at the edge of my ability, then do one or more lighter 3- to 5-rep sets. The heavy "overwarm single" is actually part of the warm-up, since it prepares me for the mental and neurological challenge of the work sets, which are still heavy but feel much more doable.