Why it's very bad for applications to start themselves automatically
Yesterday I was quite grumpy at Microsoft Teams on Linux for automatically starting itself when you logged in (without having you set it to do this), among other things. It may not be obvious why this is such a bad thing, so today I'm going to cover the two levels of why this is a terrible idea (and why it's a worse idea on Linux and other free operating systems).
On the level of an individual application, automatically starting up on login, boot, unsuspend, screen unlock, or whatever is both a resource hit and an intrusion into your work. Merely starting an application (especially a heavyweight Electron based one like Teams) uses up CPU time and power (which may be in limited supply on a laptop running on battery power), and grabs memory memory from other things. For you, the unprompted, unasked for application is an intrusion into bringing up your regular environment; you must stop to dismiss its windows and shut it down, which of course involves waiting for it to start up (most applications can't be shot down immediately from their startup splash screens). You want to get to work but instead the auto-starting application is demanding that you pay attention to it. That's a terrible user experience.
On a collective level, allowing applications to start automatically when they're not asked for is a disaster for the overall system ecology. On my Fedora Linux laptop where Teams did this, there are at least a half a dozen applications that have a plausible claim to be central to work so that they should auto-start just like Teams; off the top of my head there's a mail program (Thunderbird), a browser (Firefox or Chrome), an office suite (LibreOffice), I'm sure I have at least one IDE installed, probably the Zoom client would consider itself that important, and if I had Slack I'm sure that would be on there too. And there are definitely plenty of applications that would like to consider themselves that important.
If even half of these applications decided to autostart like Teams, logging in would be an absolutely terrible experience. If they all opened windows on the desktop, your desktop would be carpeted with unwanted windows and a machine with modest resources might take a minute or more to get them all running (and thrash itself to death) from all of the competing claims for resources. Even if they just created little applets in your system tray, it would still be very bad; your system tray would be taken over by them all and their clutter. And it would still burn a bunch of CPU, power, and so on for things that you didn't want.
Free software and free applications make this worse, because it's much easier to accumulate more applications. You'll probably only purchase so many programs, but free ones can add up far more readily. A commercial, paid office suite is only installed for people who've paid for it, not everyone with Microsoft Windows; meanwhile, most Linux desktops probably have one of the free office suites installed just because.