Why trim then clip in \includegraphics?

I use quite often trim without clip to move graphics a bit around or change the spacing before and after a graphic or get a graphic in the background:

\documentclass[10pt]{article}
\usepackage{graphicx,lipsum}
\begin{document}
\includegraphics[trim=5cm 4.5cm 0cm 0cm,width=3cm]{example-image}\lipsum*[1]
\end{document}

The original idea was that trim was just a way of adjusting the bounding box by specifying offsets from the edges rather than a new bounding box with bb, so like the bb key it adjusts the size latex leaves but if the image is really bigger than that it would over/under print the surroundings. clip clips. It's a long time ago and i can't remember all the discussions but probably the fact that clipping isn't the default is related to the fact that not all the important dvi drivers at the time could clip at all.