Its for the beading between the 2 panes of glass that make double glazing.
Its either the trim at the edge that holds the dessicant to absorb moisture - which is why there are little perforations -or if you have mock georgian effect, its the cross bars that form the pattern.
It is intended to make double-glazing units more secure.
They used to be beaded from the outside, which meant a would be burglar could just remove the beading and then the glass.
That is no longer possible as the units are placed in the frame from the inside would be quite difficult if not impossible to remove from the outside.
Hi, Mattie is right except if someone has external beading it maynot be so easy to get the glass out. The external beading could be a shuffle bead where the bead interlocks with the frame. Unless you know what you are doing you have to take a hammer and chisel to get the bead out.
Some window with external beading have the glass stuck to a security tape making it very difficult to get the glass out.
However Mattie is quite right. Some of the older windows are easier to break into.
Some insurance companies specify internal beading on new build house as they do provide more security. Rgds Al
From my days at Coldseal I also recall that internal beading is specified on high rise buildings where it would be very difficult to get access to the external side of the frame.