Just promoting their own efforts: and mostly in reaction to nintendo's playchoice: Arcades were sega's domain and they were looking to invade nintendos home market - looking at how Nintendo beat atari, Sega thought they'd do the same: beat them in the arcades, port arcade to home and beat them there too... Nintendo had their home games in the arcade, Sega would do the same - showing up the 8 bit Nintendo efforts and promoting their home games at the same time. That was the plan. And it worked pretty well even when the home market evolved... Although of course nintendo soon hit back concentrating on that growing home market that they understood better.
Although mvs is often seen as THE multi game system (because it was frankly bigger and badder it hit harder for longer) both Nintendo and Sega kind of beat them to the punch with the idea.
If anything the stv was supposed to beat out the MVS, but with capcom pushing cps it was outclassed and denied the 2d library it required. And segas marquee arcade efforts were all about premium 3d hardware which was quickly evolving and left the stv out in the cold even at launch.