It's really simple, actually, as the title says.
When you navigate to an activity from somewhere else than your personal feed (from a segment performance list, from search, from...), it's very hard to navigate to the adjacent activity. The computer could easily do it, but we can't.
Occasions where this would be desirable include a wide range of things, from formal triathlons, where the different sports are usually tracked as separate activities, to state races, to epic bikepacking trips, to something as mundane as morning/evening for those weirdos who insist on tracking their commutes, separately.
I'm sure that specific use cases like triathlons or vacations have been feature ideas before, but a general previous/next would solve them just fine, without any detection troubles)
