We "store" the logo
It is very important to design a logo that will stay in the viewers' minds. At the next gaze of the logo, the beholder should remember that he has seen it before.
This way, the beholder creates the association between the logo and the corresponding brand, and possibly with the feelings that this brand conveys.
If a logo is well-introduced and has the quality of being easily memorable, the ideal case is that even a part of the logo will suffice for the viewer to relate it to the proper product or the proper company.
If this is the case, the logo is very well-suited for branding.
Two examples: The logos of McDonald's and Coca-Cola. Small parts of these logos are enough for us to know the company that the logo belongs to.