The SLAM procedure that elevates chlorine levels is most specifically designed for the most common situation of algae growth (which often clouds a pool) and most algae is readily cleared by 1) killing it with elevated chlorine levels, particularly higher SLAM levels to get through algae clumps and 2) circulation/filtration. This doesn't need clarifiers of flocculants. It definitely needs chlorine or else the algae can continue to grow while you are trying to clear it away.
However, persistent cloudiness after other failures are ruled out such as channeling in a sand filter or holes in a cartridge filter or poor bottom circulation usually means that the particles are finer than the filter can capture efficiently. In these situations a clarifier or flocculant is about the only way to deal with it (if DE or equivalent fiber product in a sand filter isn't sufficient) other than water replacement.
Chlorine itself does not clear cloudiness. The purpose of chlorine is to kill algae. It is circulation/filtration that removes dead algae or other material from the pool. There is some microflocculation effect from chlorine, but it isn't the main way to clear a pool.
Now if one is going to be using a clarifier or flocculant in these less common situations, one must use the right ones. Some don't work well (we just saw that recently). For clarifiers, we've seen good results from GLB Clear Blue, BioGuard Polysheen Blue, and one other I can't remember right now while for flocculants it's OMNI Liquid Floc Plus, BioGuard PowerFloc, or most alum floc products. However, using clarifiers or flocculants is very tricky as one can overdose and make the problem worse. Also, some metal sequestrants (anionic polymers) are incompatible with clarifiers and some flocculants (cationic polymers) and will produce a precipitated mess.
Pool services will often use clarifier or flocculant products because they don't have the time to maintain a SLAM and wait for circulation/filtration to clear a pool more slowly. So for them the extra expense is worth it and they usually settle on a product that usually works and they get a sense for how much to use in different situations. Nevertheless, they also have the same problems or trouble when it doesn't work and we've seen reports of that on this forum (i.e. my pool service tried a clarifier or flocculant and my pool is still cloudy).