Using numbers instead of letters is a way for video game companies to gear their games towards the online generation who have created a "Leet Speak" (1337 5P34k) language based around substituting numbers for letters. Sometimes, the numbered representation is only for marketing and not the official title.
Replacing letters with numbers also applies to replacing entire words (e.g. "4" instead of "for"). The use of numbers for letters can also signify which iteration in a series the game is. For example, Driv3r is the third iteration of the Driver franchise. Occasionally, games will only use this concept in certain regions. For example, the third entry to the Wipeout series was billed as " Wipeout 3" in North America, but " Wip3out" elsewhere. Other examples may refer to features as opposed to numerical designations; Ratchet & Clank: All 4 One is not the fourth entry in the series, the 4 in the title refers to the game's emphasis on its new four-player co-op mode. Similarly, Crush3d is only the first sequel to Crush, with the additional characters referring to the fact that the new game is in 3D.