People share old photos to social media for many reasons including
1)Identity – establishing identity
2)Connection – with people locally/regionally or to a regional area (place they grew up) – old friends/family or to identify old friends/family
3)Education – learning about and participating in history
4)Nostalgia
5)Research
6)Organize the world/Understand humanity
7)Art/Creative value (some of just like to look at photos)
Research on the use of old photographs on social media supports these. Looking at old photos of ourselves or places we lived helps us define and retain our identity, which is especially true for those with cognitive issues.
Articles about looking at old photographs
http://www.smithsonianeducation.org/idealabs/ap/essays/looking.htm
http://www.psychologytoday.com/blog/snow-white-doesnt-live-here-anymore/201112/what-can-old-photographs-and-letters-reveal-about-ou
http://www.psychologytoday.com/blog/unfinished-business/201009/windows-our-psyche