Mosses tend to terrify some of those among us who own small gardens. On the other hand the mosses are a paradise for the water bears. In the literature we find quotes of population densities up to 22,000 individuals per gram of dried moss (see e.g. in Ernst Marcus:  Bärtierchen. Jena 1928, p. 18).
Very old publications indicate that the water bears are found most typically in roof gutters. In his   first description of   Macrobiotus hufelandi   Oken mentions the existence of water bears in roof gutters but doesn't forget the roofs themselves. Later authors simplified this information in so far as they skipped the roofs and mentioned the roof gutters only.
E.g. Ludwig Schmarda writes in his biology textbook "Zoologie" (2nd ed., Leipzig 1878) about the tardigrades:

We can easily understand why the water bears can be found in roof gutters. It is a kind of accident. Strong rainfall tends to carry away the water bears from their favourite places, the moss cushions on the roofs. The water bears then end up in the gutters as homeless animals:

Meanwhile many people on earth live in houses covered by new, sometimes glazed tiles where the tardigrades cannot live anymore.

What are the advantages of those mosses on old roofs?

Let's have a glance at the properties of the mosses first. We can perform a simple experiment in order to understand.
Just take one of those dark, dry moss cushions, e.g. of   Grimmia pulvinata  , from a wall exposed to full sunlight and weigh it. By means of a letter balance we can tell that the weight of the moss cushion shown below in the figure is a little bit less than 5 g.

Now we flood the moss with deionized water or tap water until it is saturated. In the experiment shown here it took about seven minutes until the moss was fully soaked by the water. We let drop down the remaining unbound water and weigh again. Now the balance reading is 24 g. We conclude from this experiment that the moss is able to take up its fourfold mass in water.

And, possibly even more strange, we notice that it has not only grown in size but that its colour has turned to an intense green again. Before it had an ashgrey colour, like dry brushwood. You need lots of light in order to present this very dark green colour on the crt:

Furthermore we note that some part of the moss seems to behave differently. Apparently this part is no more alive and therefore cannot return to the green state.
Now we have lots of water caged within the moss. Due to the many leafs the outer surface area is great and therefore we will always have enough oxygen in the water.
In the last issue of the Water Bear web base we have learnt that the water bears cannot  breathe  actively and that the oxygen therefore has to penetrate into their tiny bodies just by means of diffusion. So a well-ventilated system will provide an optimal environment continuously saturated with oxygen.
When looking at moss cushions on earth overloaded by water for several days we will find out that the number of bacteria and funghi tends to increase rapidly and that the moss will become smelly after some time. The tardigrades within those too wet moss cushions are threatened by the bacteria:

Though many of us are afraid of bacteria as well we will have to admit that the scale relation is much more favourite for us humans: From the perspective of a tardigrade a single bacterium has the size of a hot dog sausage - which is really a terrible situation!

Due to their water storage capacity the mosses offer rather long periods of constant humidity to the water bears. In contrast to the mosses on earth the mosses on roofs will receive lots of water during a rainfall but will never bind more water than what can actually be retained by the moss plants. For this reason there is no dammed up water within the mosses and no stagnant humidity. From time to time the moss becomes even completely dry and the number of bacteria will become smaller. So after the next rainfall the tardigrades will have a fair chance to revive in a clean environment without any 'hot dog' bacteria.

It is really a pity that the number of old roofs tends to diminish worldwide ...