Andy Brownsword has it together:
It’s a legacy pattern, and thankfully it’s rare to see these in the wild nowadays.
The legacy
OUTER JOINsyntax (*=and=*) which used to accompany these was deprecated, and finally removed in *checks watch* SQL Server 2012, so that’s one less reason to see the aging syntax.
Every time I see this format, I despise it. Andy explains exactly why. We’ve had better options for more than 30 years, yet people still choose to write code this way.