[biggerfisch]

No, not really. Filtering out atmosphere isn't that hard for some (most?) spectra that we care about. The benefits of a bigger mirror are more about resolution than signal quality. There are physical limits to the size of a distant object under observation that can't be replicated just by more/cleaner observation time.

[DiogenesKynikos]

Not just resolution, but also collecting area and weight. If you think of a telescope as a bucket that collects photons, you can build a much bigger bucket on Earth and get a whole lot more photons. That allows you to see fainter objects.

Weight allows you to put many more scientific instruments on the same telescope (fiber-fed spectrometers, intregral-field spectrographs, imagers, coronagraphs, etc.) In order to put an instrument on a space telescope, you have to miniaturize it and make it light, and you have to decide before launch what instruments to put on the telescope. In contrast, on the ground, you're not nearly as weight- and size-constrained, and you can upgrade the instruments on a telescope every several years. There are decades-old telescopes still in use, retrofitted with modern sensors.