Signs and banners are physical representations of a mental phenomenon. What I mean by this is that when a person creates a sign what he or she is doing is transferring ideas (which are a mental phenomenon) onto a physical object like a piece of cardboard or wood. Once the idea has been transferred onto this physical object it now has a physical existence outside of the mind. Once this happens, there is no need to continue to store the information in the mind of its creator. This is not to say that the creator of the sign can actually delete the information from his or her mind. It will of course remain in some lingering form for a period of time. But what the sign does is it allows the idea to remain in existence should that idea disappear from the mind of its creator.
The other function of signs and banners is that they continue to act once the idea has been transferred from the mind of its creator onto the physical object that is the sign. What I mean by this is that the purpose of the sign is to convey an idea to a person who sees this sign. The sign might contain an instruction or an advertisement (which is an instruction or suggestion to purchase something). If the person who created the sign was charged with the responsibility to give this instruction to other people that would require that particular person to remain in place in order to give the instruction. However, once the instruction has been transferred to the sign there is no need for that person to remain in place. The sign will do the work of the person.
This might seem like a simple concept that would not require a three hundred word blog post to convey. To a certain extent that is very true. But there are further subtleties that exist to this exploration that perhaps can be the subject of another post.