Opinion

Why would we use text in illustrations? Aren’t illustrations meant to work without text?
A photographer friend told me once that a photograph with added text is a bad photograph. The effect is too easy. Text manipulates the interpretation of the image and weakens its visual impact. A photograph should tell the story with pictorial power only. 
It took me a while to get his point, but it is ever since nested in my mind. 
There is a conflict of interest between words and images. Words trigger mental images, you can compare this with the way poetry work. That is the strength of language. Words that appear as images also trigger mental images. Ed Rucha's paintings creates this effect and Laurence Weiner's textual installations do too.
Illustrations with text serve a specific purpose. Does the text comment on the picture or does the picture comment on the text? You can talk about an illustrated poster, or an illustrated essay. A cartoon is a visual comment, but usually needs a written caption. Text creates the context of the image.

A picture sends visual signals. An image with multiple visual signals generates a visual message. If we understand their symbolic meaning, we can “read” them. Written language can be much more explicit than a visual signal. Text creates the context of the image by manipulating its interpretation and shifting its intention. The image excludes any other visual exploration other than the image itself and adds meaning to the text.
The visual rhythm, information hierarchy, reading directions, in both text and image are key aspects in arrangements. Whether harmonic, atonal, synchronized or not, its final result is a composition of all visual signals and written elements. 
Using text in or with images is an ongoing challenge without a premeditated solution. As far as I am concerned its process has to work from the inside out. Triggering imagination with words works similar as with visual elements. They should not explain but seduce to participate.
Any way, that is how I like it. I look for universal qualities that raise language and image to a higher level. An image with text is not an image at all.