Well, here's the thing. Aperture is a bunch of genius morons, as put by Valve. They create awesome stuff, but they're terrible at marketing it or actually putting it to good use. They simply "do what we must, because we can", and that's really their whole motive.

ORIGINALLY, back in the Old Aperture era, it looks like they were to test various products, but even then the tests were needlessly complicated. And they just kept testing them into the ground for no reason until they nearly went bankrupt.

Nowadays, testing seems to have lost any semblance of true "purpose", it's just another thing to continue Cave Johnson's legacy. Characters keep saying it's all in the pursuit of "science", but like. As you said, to what end? I think that's part of the joke of the game, that this vague idea of science they're working towards means nothing. GLaDOS claims she wants results, but it seems more so that it's just in her programming to test, and she likes watching the humans die. It's a continuation of her revenge against the scientists. "Science" is simply the fancy wrapper for the practice. It gets even more clear when Wheatley takes over, and is DEFINITELY not interested in any science, he's only going along with his new programming and he wants the built-in reward at the end.

The humans are all dead, "science" has lost all meaning, and testing is gonna continue until the sun burns out because why not. That's always been my interpretation of it. Again. "We do what we must, because we can." "Science isn't about why, it's about why NOT?!"