In the days when the great collectors had thousands of paintings, drawings, and prints, they would put a stamp, a seal, or a mark on every piece in their collection.
The same was done in the royal collections.
Sometimes, an inventory number, or collection number, would be added.
These collection marks have been recorded. There are thousands of them. Researchers such as Frits Lugt spent decades copying, identifying, and listing these collection stamps.
When a painting, a drawing, or a print bears one of these marks, it means that at some point in time it was in an important collection or in a royal household.
Many of these collections and royal holdings are well documented in old archives and historic papers, and they frequently provide information on where and when the pieces were acquired, who executed them if they are not signed, and sometimes additional information such as title or date of creation.
These marks are proof that a work of art existed in the past and can by themselves replace the need for provenance research.
For these reasons, stamps, seals, and marks are extremely useful because of all the additional information which can be derived from them.
Even in the absence of old inventory records or royal archives, it adds value to a work of art, if it used to be in a prestigious collection, a royal collection, or the household of a prominent or historic figure.
Many of these marks are small, rubbed off, discolored, and difficult to read and identify precisely. Wax seals are dried out, discolored, fractured and frequently have missing parts. The way to deal with them is to take close-up pictures and magnify them in super size. Image processing software can then be used to enhance contrast, brightness, intensity, or to draw outlines, in an effort to decipher them.
Once this is done, it is a matter of locating the mark or seal in reference books and then to research what is known about the collection it used to belong to.
It is not always possible to identify the artist from a stamp, mark, or seal, but at least it provides a solid starting point with a date and a location for the collection, if it is a recorded one.