I was looking into the schematic of the Arduino Uno Rev3 particularly the built-in on board LED (connected to pin 13) and was curious about what was happening.
Here you can see pin 5 of port B being connected to what appears to be a comparator which is in feedback with its own output.
Why not just connect it via a 1K resistor to yellow LED L?
That would load the pin significantly if it was configured as an input.
Using half of a dual op-amp as a buffer is a reasonable design choice, when you need the other half elsewhere. Might not be optimal in all cases, but it's not unreasonable.
Another choice might be to use a 2N7002, with or without a series gate resistor and an LMV321 single op-amp for the other requirement.
The LMV358 is an opamp, not a comparator, and that's called the "voltage follower" configuration. It's functioning as a buffer so that SCK isn't loaded directly by the LED.
