The manager is excellent for ordering clothes production. You don't want to use all your cloth on socks by setting it to R, but if you want say 60x (socks shoes trousers tunic hood cloak gloves) that would take 42 workshops, and you'd need manually queue each item. Its much easier to do this from the manager, it would take about 14 orders (max is 30 items per order). Furthermore, if jobs are canceled due to lack of materials, the manager will remember and keep queuing those jobs instead of you trying to figure out of 5 helmets were made or only 3 before you ran out of steel. Lastly, accessing the manager and ordering items is exceptionally fast compared to finding the workshop and then ordering the item from the menu.
The manager is NOT particularly suited to situations where you care about quality and you have workshops for high skilled workers and others for low skilled workers, or if you care about the input materials (and there is variance in the input materials). Masonry is the prime example. If you have a few workshops for block making restricted to low skilled dwarves, and a few for high skilled dwarves to make furniture out of ore-rocks or rocks of a particular color, the manager won't be useful to you. Another example would be in metalsmithing. If I have 9 forges, but only 1 legendary metalcrafter with that job enabled, ordering 90 brass crafts will block all the forges without speeding up production. Similarly, if you want to decorate all your fresh armor in brass and electrum, you might have a stockpile for armor set to give to one of your multiple workshops. In this case, the manager would not be helpful.
however, if you need like 30 floodgates fast, and don't care the color or quality, then the manager is right for you!