When one is trying to express the action of filling something, there are many different synonyms that can be used for the word "fills for." For instance, one could use the word "satisfies," which suggests that the item being filled is one that is lacking or empty in some way. Another possible synonym is "crowds," which might be used to describe the action of filling a space with people or objects. Other options might include "packs," "loads," or "stuff," each of which emphasizes the act of filling something up with things of various sizes and shapes. Ultimately, the choice of synonym will depend on the specific context in which the word is being used.