Television series try to amortise their costs over the course of a series, budgeting more for openers and finales (especially in the first season, when they have to construct the standing sets). As a result, they do what they can to save up money for cool stuff like location shooting, complicated visual effects shots, extra days of production, and the occasional really big practical effect like an explosion. That's why once a year, you'll have an episode where Our Heroes are either trapped in a bank vault, or held hostage at gunpoint in their homes or place of business, and may also only feature their contracted cast with few or no other speaking parts. "Duet" in DS9 is an example of one of the best bottle shows ever, and featured extraordinary performances from the cast and particularly guest star Harris Yulan. RTD's "Midnight" on Doctor Who was as close to a bottle show as nuWho gets, and it turned out to be a fantastic claustrophobic one-act play (not unlike the very best types of audio plays) that had minimal scary rubber monsters, and Twilight Zone levels of paranoid and fear and human drama (and was shot in parallel with "Turn Left", much as SGA split the cast last season to shoot two eps in parallel on different sets at the same time).
I love bottle shows. Love them. They are my most favourite thing in the whole entire world, after sushi and dark chocolate covered candied orange peel. I love them better than pie. Like a fat kid loves cake. Like a man loves a woman. Like occasionally a man loves a man. I even love it platonically, without puppets. Because as part of their structure, they often involve characters spending a lot of time talking to each other. I know talking heads are supposed to be the kiss of death on television, but to me, any opportunity for character development and furthering characters relationships where it's not superfluous to the plot, but is in fact integral to the plot makes me GLEE.
Granted, if I am wrong, then ignore the practically-vibrating-out-of-my-skin-wit
P.S. also? TINY TINY ROBOTS!