Two components dropped from logic board, what are they for?

Those are capacitors. Specifically 6.3 Volt 150 microfarad capacitors.

I won’t get in too much depth about their overall function (unless you want). But in this case your iMac probably will not turn on without them. It might. But I am rather doubtful. Not only does the schematic note them as “critical,” they are part of the circuit used to generate a specific voltage rail that is necessary for power on.

Sometimes capacitors can be left off, it depends on what their purpose in the circuit is. It won’t do any hard to try and boot it up without them, but I don’t expect you to get far.

Luckily, as far as components that could have popped off the board, these are big enough that soldering them back (or getting replacements soldered on) shouldn’t be too difficult with a little solder know how. It looks like they might have just had some brittle old solder joints that decided to give up the ghost during the repair. If the capacitors themselves don’t seem to be bulging, or damaged in any way, you can probably just solder them right back on without any harm.