Lace can not hide the knot in the same way a monofilament base does. Bleached knots, usually only done at the lace front and it helps create the illusion that your hair is growing out of your head.
If your hair color is Black, Dark Brown, Medium Brown or Brown, then knots that hold the hair onto the base will be changed from black to brown and will be much less visible. Single knots such as those used at the front edge will be be very hard to spot.
The bleached knots can also be bleached in an area of the unit that is poly coated.
Depending on your hair color, the bleached knots will be light brown or blonde.