I understand that in most European countries, glider flying in cloud is
prohibited (though occasionally done by some lawbreakers, I have heard).
I have seen references to it occasionally in the USA so presumably it is
legal there. In the UK it is permitted, outside controlled airspace.
Most often this means climbing not just up to the base of a cumulus
cloud which has been formed by a thermal, but continuing on into the
cloud to get higher. It is sometimes possible to achieve cross country
flights by this means which are difficult or impossible if each thermal
has to be left at or lower than cloud base.
Having climbed above cloudbase, if one comes out of the side of the
cloud, another may be in the way en route, and one option is then to fly
through it rather than deviate round it.

Wave flying is another scenario - cloud may form around the glider or in
front (up wind) of it and so flight through it becomes unavoidable.

Cloud flying is potentially dangerous and should only be attempted after
suitable training, and with appropriate instruments. It is said that the
inexperienced can lose attitude and control in as little as 45 seconds.
Those who have not experienced the disorientation do not believe it can
happen that quickly, and a few have carried their disbelief into
practice with fatal results. Loss of control almost always means
entering a spiral dive, pulling high g and gaining speed. In modern
gliders, acceleration can be very rapid, the brakes or spoilers are
often not speed-limiting, and the wings come off. The last fatal gliding
accident of this sort in the UK was to one 10,000-hour (IIRC) pilot with
some experience of cloud flying a few years ago - the barograph trace
showed the glider broke up at about 10,000 feet at high speed, in a
modified Slingsby Vega with new tips.

Chris N.