So then, why is so much piled into these low cost dso's? Who is it all for?
In the last couple of decades so much has changed in the electronics that that the role of oscilloscopes has changed also significantly. Naturally one thing has not changed: An expensive dedicated test equipment is typically better in what it's dedicated for (cheap ones not necessarily).
Nevertheless a decent DSO can be a substitute for some of those dedicated equipment (especially for hobbyist if they don't have very demanding requirements from them in certain areas, or big $ pocket which is actually kind of the same thing).
Thus these low cost DSOs are mostly for hobbyist (in my country I know that some repair shops are using them). There are couple of equipment that one cannot avoid even as hobbyist and DSO is one of them. For DSO vendors those extra features costs from little to nothing. Just SW on existing HW.
Although the main role of looking at waveforms like you wrote, but not only that.
This has to do a lot with how electronics has changes in the last couple of decades. Back the majority of the stuff was analogue (well with some exceptions like 74 series of ICs and 8051 micro that arrived gradually).
Deep down naturally still everything is analogue, however from practical purposes real analogue stuff (where core functionality is still analogue) has remained only a few places.
Today most of the signal generation and processing has transferred to digital domain. (This does not necessarily mean DSPs as MCUs have become quite powerful, many times MCUs are used instead). For fast logic CPLDs and FPGAs are used even by hobbyist.
Thus we have purpose made chips that communicate via some interface (like SPI and SPI can use fast bitrates...) and generic chips where logic is defined via some HDL language (and where really fast communication is needed e.g. LVDS is used).
Naturally if you work with analogue audio and periodic signals (and couple of more areas) none of these change impacts you.
However most of us that has something to do with modern electronics are very much impacted.
Naturally even in these realms some signals can be made periodic. However sometimes it becomes increasingly hard, sometimes downright impossible. E.g. although I've occasionally did, I'm generally unwilling to change HDL code to make something periodic as the synthesized HW will be different and glitches invisible because of the difference made.
Spectrum analyzers are for looking at the frequency domain.
Sure. However DSO has the high impedance input and can provide better resolution for lower frequency ranges. Potentially you may have probes for the DSO that even if you could have for SA, you'd rather not duplicate them for pure economical reasons.
I can't emphasize enough the probing part of this story.
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Logic analyzers are for looking at digital signals.
Sure again. Logic analyzers have the same probing issue again if you work with fast signals (and even couple of $ MCUs can produce such fast signals nowadays that without proper probing your analyzer fails to see or even worse potentially corrupts it). This has been recently discussed in the forum and it worth checking these 2 videos (sorry for reiterating again):
[001] Sigrok and Logic Analyzers
[009] DSLogic Logic Analyzer Review and Teardown
So while some vendors tried to address the problem, and certainly there is generic solution in the nxk$ domain, if the number of signals that you need to check does not exceed what your DSO is capable of DSO is actually comes very handy in case of fast signals.
Better waveform display than an analog oscilloscope?
I guess it heavily depends on how you define the term better.
I'd rather look for better triggers as there is no replacement for that.
So as short version: killer use cases for DSO, single-shot events (including protocol decoding), probes that you have available for fast signals, FFT and so on.
I guess everyone who has switched to DSO (almost everybody based on the offerings of the oscilloscope vendors) has at least one good enough reason, since DSOs tend to be significantly more expensive than the analogue oscilloscopes that you find.