When this happens, a programmer will often use or create a placeholder, meant to be replaced at a later time before the project is published, although this doesn't have to be the case.
These assets can be made for quick testing of a feature, for behind-the-scenes reasons, or even fully intended to be shown to the end-user.
The art can be made to look intentionally crude, so as to alert play-testers that a certain part or mechanic is not yet finished.
In indie games, programmer art is often the norm as small-time developers rarely have dedicated artists or budgets for professionally made assets.
It is a recurring trope for programmers, who are often believed to be logical-minded, to have little experience with or interest in creating art.