Why can you put speed limitations in the "legs" page in the FMC just in combination with an altitude?

Question

Because on the other side, you can put an altitude in it without giving a speed. So, why is that?

Answer 1

What you are actually doing is building a visual aid for making crossing restrictions.

That comes in very handy when given a clearance "Cross ABC VOR at FL 280 and then maintain 280 knots."

It does not actually enter into the aircraft's fuel burn calculations (go into your defaults to change that value).  All it is really doing is telling the FMC to draw a picture on your MFD as a reminder.  It will NOT actually do it, its up to you to actually move the speed bug and slow the airplane down yourself.  Hopefully, having it drawn on the screen right in front of your face will remind you.  If you feel you must enter an altitude, you can always just enter your current Flight Level.