given enough time, silicone will lose it's 'grip' on glass, and peel off w/just a tug-i've seen it happen on tanks less than two years old, that weren't prepped correctly-just takes longer for those done right
i check the silicone on all my own tanks yearly, running along the thinnest part of where the glue seam's edge just start to grab the side of the pane (that really thin layer that spreads out from the actual corner, just for peace of mind
the reason for using potato, and not one's finger, is that the potato is moist, and won't allow the silicone to stick to itself, making a nearly perfectly smooth seam
even w/a wet finger, you will probably have to continually wipe, and rewet it, and the result will be far more sloppy, and uneven
the fit of the arc of the curve of the potato forms the seal seam perfectly-draw a 90° angle, then draw an arc, starting from one side, bowing into the corner, and finishing to the adjacent side
that will give you a bird's eye view of the bead shape as a result :wink: