Scanners, microscopes and print workflows produce TIFFs of tens or hundreds of megabytes that will not attach to an email or display in a browser. JPG brings them down to a size you can send. It is lossy, so keep the TIFF if it is an archival master.
JPG has no transparency, so see-through areas of a TIFF are filled with a solid colour, white by default; you can change it under Options. The defaults suit most files. Under Options you can set the quality, aim for a target file size and resize the result.
Only the first page of a multi-page TIFF is converted. Camera metadata such as EXIF and GPS location is not copied into the JPG, and the photo's rotation is applied so it stays upright.