There are two aspects to it.
The first one is brand reinforcement as the appearance of a well known brand such as intel or Microsoft helps authenticate the laptop. I know these details can be present on the system as well but these stickers help authenticate and reinforce a brand during displays as they highlight the key offerings.
Another obvious reason is brand visibility. Though one could argue that consumers are computer savvy and would know what they want in a computer, the visibility of a brand icon on some else's computer can lead to recognition when you decide to buy a computer later and the same icon is there. The visibility of the logos also helps in reinforcing the brand to users.
However the legal reason is that Original equipment manufacturers (OEM's) require companies to display these stickers as part of their tie up with them.To quote this article from the NY times
There’s big money involved. Intel, Microsoft, Skype and whoever else
is represented by the stickers actually pay the computer companies for
the billboard space. That’s why H.P., for example, would tolerate
gumming up its laptops’ good looks with crass ads. (Apple refuses to
put Intel stickers on its computers, even though there’s Intel inside.
In doing so, it leaves millions of dollars a year on the table.