Heyo!
Goodmorning. I know, it's the afternoon, but hey I just woke up, that's how life is.
Okay, so,
Companion cells are cells that are necessary for the function of the phloem tissue; they actively onload sugars and offload them, too. You're fine with onloading, so, offloading it is.
At the sink, there is an inflow of sap through the phloem: it has liquid content as well as solute content, i.e. water, and sugars such as sucrose. To get this sucrose to the cells that need it, the sap can't just directly flow through cytoplasmic pathways/etc to the cells, that's a heck of a lot of sap, and all you'd have is a hydrostatic pressure gradient, not really as much a concentration gradient, so the companion cells offload the sugars inside the phloem through a partially active partially passive process: these sugars accumulate in the companion cells and through various diffusive means make their way through to the cells that need them.
Another thing worth noting is that if phloem just delivered sap to the cells and companion cells didn't do the offloading, there's no guarantee the phloem sap would reach the base of the plant with high enough pressure to deliver anything, I mean it might as well just all get pushed by the pressure into whatever way the cells would get it without this companion cell offloading system. Either ways, stuff just doesn't work well without them doing it.