What Saturn does in a chart
Saturn is the part of the chart that answers one question:What do I have to work at over time? It describes structure and commitment — not events, and not outcomes.
What Sagittarius does to it
Sagittarius is a fire sign, so it moves on impulse and enthusiasm. It is also mutable, which means it adapts things. Put those together with Saturn and you get works at sagittarius themes — roaming, candid, hopeful, applied specifically to structure and commitment.
Sagittarius is ruled by Jupiter, so this placement tends to be coloured by growth and belief as well.
The useful version
Placements are not verdicts. This one describes a default setting, not a limit — the things listed above are what you do when you are not thinking about it, which is exactly why noticing them is worth something.