Quite simply, enclosing long URLs that wrap across lines in angle brackets enables email clients to recognize the entire broken string as a single URL and to make it clickable. This has been true for a very long time, which is why TidBITS started adding angle brackets to all URLs (to do it for only long ones would be inconsistent) back in 1995. To this day, most email clients I'm aware of won't parse line-broken URLs properly unless they're wrapped in angle brackets.

http://db.tidbits.com/getbits.acgi?tbart=1571

As far as copying and pasting goes, the return character usually had to be removed manually, so it was generally considered easier to click a URL and open it in a Web browser for verification, then copy from the Web browser address field.

cheers... -Adam