The most common reason is an online form that rejects the attachment because it exceeds two megabytes, or a mailbox that won't let the email through. Photos from modern phones easily weigh five or ten megabytes each, well beyond what is needed to view them.
The quality slider controls the trade-off. Around 70 per cent the difference is barely noticeable to the eye and the size often drops five or ten times. Below 50, smearing starts to show around edges and in flat areas.
You can compress several images together: if you select more than one, the result comes in a ZIP archive. The output images are JPG, the format that compresses photographs best.