Exclusive
The term exclusive is most often used in reference to a toy (or other item) that was only available at a specifc store or venue rather that at standard retail in normal assortments.
Store Exclusives
Though store exclusives in Transformers date back as far as 1989's K-Mart-only releases of the "Classics" Bumblebee, Grimlock, Jazz and Starscream, the practice saw little use in the US until Kay-Bee stores got its own exclusive mini-line Machine Wars in 1997. Store exclusives tied in directly to the main retail line, however, remained minimal in the following years, until the success of the Robots in Disguise series caused a boom, with the "big four" chains (Wal*Mart, Toys 'R' Us, Target, and Kay-Bee) each getting their own Transformers avialable only at their stores. Since then, store-exclusives have persisted, though with the creation of the Universe series, most have fallen under that line, even well after the Universe line disappeared from standard retail, and even with toys whose backstories place them quite explicitly in other continuities.
Convention Exclusives
Convention exclusives have a more consistant history, however, starting back in 1994 with BotCon 1994's Generation 2 Breakdown, which had been slated for normal retail release with the rest of the team, but ultimately cancelled. The following year saw the first convention-exclusive toy redecoed specifically for the convention, the Nightracer.
From then on, each official Transformers convention has had at least one exclusive toy... in fact, as time has gone on, the scale of convention exclusives has only expanded, with multi-toy packs, larger molds, and even retooled items.
Why Exclusives?
God hates you and doesn't want you to have toys.
