The reason is almost always the same: an attachment that won't go through. Mailboxes typically stop at around 20 or 25 megabytes, government portals often much earlier, and a twenty-page scan made at high resolution goes past them effortlessly.
Recommended compression works on the images only: the photos and figures inside the document are reduced to the resolution at which they actually appear on the page, then recompressed. Text, fonts and vector graphics stay exactly as they were, so the text can be selected and searched just like before. While it's at it, the tool rewrites the internal structure more compactly and removes duplicate or unused objects. On a PDF full of photos the size often drops five or ten times; on a text-only PDF the gain is small, and in that case the tool tells you so instead of handing back the same file.
Strong compression is plan B: it redraws every page at the resolution you choose and saves it as an image inside a new PDF. It shrinks even PDFs without images, but the text becomes an image: it can no longer be selected or searched. It is the right trade-off for scans, which are already images and have nothing to lose, and for any document that only needs to be read or printed.
As a rule of thumb: use recommended at 150 dpi; if the file is still too large, lower the quality or the resolution, and switch to strong only as a last resort.