sometimes CC64 is used for simple convenience instead of CC68.
Using the sustain pedal of the piano to play legato is like cheating!
Sorry, I'm joking... Sort of. But many pianists may say something like that, in a very serious tone.
The point is that some instruments, like the acoustic piano, are not really melodic instruments. Each key is attached to a hammer and when you press it down it strokes one or several metallic strings, producing an audible "bang" with percussive quality. It doesn't matter that the hammers are made of wood, and covered with felt. It could be worse, but still... Playing legato on the piano is quite difficult.
Compare to the violin. Legato is performed by keeping the bow movement on the same direction during the legato phrase. When the bow movement direction changes, there is a distinctive rubbing sound that a violinist wants to avoid when playing legato. Wind instruments also have artifacts at the sound attack, produced by the player's mouth.
Synthesizers create the notes using an envelope generator, usually with four stages: attack, decay, sustain and release (ADSR). In legato mode, the notes do not perform the whole attack and release phases, which are overlapped with the adjacent notes. In Fluidsynth, you can choose among two legato modes using the "setlegatomode" shell command, and it accepts CC68 as the on/off switch. But it also depends on the soundfont, and each program within the soundfont. The tutorials on the "polymono" directory suggest the General User GS soundfont, and its (beautiful) flute program to illustrate the legato modes, which are quite convincing. But you may use the program 0 instead (acoustic grand piano) that sounds quite unnatural in the default legato mode, because sometimes the hammer blow is almost removed from the sound. If you use another soundfont and other programs, the results will be different.