Hmm. Yes this SHOULD be in the wiki. I'll do my best.
Some 4-5years ago, S60 (version 2nd Edition, FP1/2 = Feature Pack) phones not as advanced today but still with MANY applications had a particular issue. Security of applications (by Joe Rock the self educated coder in university, or large corporations like EASports that employed TEAMS of them) was at an all time low. Especially when the NGage & NGage QD handhelds debuted with such great games, ppl didn't like paying $30US for them and hackers found a way to emulate the /System/BIN contents of the phones Firmware folders (C, E, Z drive I'm not sure which). Certain files that permitted NGage games to work on only the NGage handhelds where soon hacked or the games themselves and the hardware (cpu), and RAM in similar phones allowed these hacked games to work. IMEI restricting didn't seem to work too well because it relied on a file that the software needed. Applications soon after where also figured out and you can IMAGINE how EASY it was to have hacked games/apps on just about ANY S60 2nd Edition phone you wanted.
This caused a LOT of pain in the industry and thus Symbian Signed was created - not just for S60 3rd Edition, but also for UIQ 3. However, its taken a little while for Hackers to do the same, copyright infringement (sourcing, using, sharing hacked apps/games) is NOT as abundant as it was several years ago. Part of that is the current state of the world economy fairing better than it was several years ago (US consumers excluded as a whole; but there is a LOT of iPhone App Store purchases - over 30Million US$ worth).
Thats the best of an initial explanation of WHY Symbian Signed exists I can give you. HOW to Sign applications for use with your individual phones' IMEI I have no clue as I'm searching for that info as well. Maybe others such as Ricky, Marc, JonT, etc can explain it better - they have switched amongst many S60 3rd Edition phones.