When you're out jogging, your phone might stop working because the display came loose. Or if it falls off your bed at night---that kind of thing. So open it up and plug in the connectors. Personally, I take my own phone apart so much wherever I go for demonstrations, I never keep all that stuff on there. My battery comes unplugged, but not so much the lcd/digit/prox flexes.
Order it from china for 50 cents on eBay and just let it take a month to get here.
Professionally, I always fill in those small parts so that phones leave with all the right pieces. It is just a matter of personal preference.