1.) Wheat and Barley go through a ripening stage in which the plant naturally dies, artificially killing the plant before this happens would result in unripened grain and unusable straw. Different varieties ripen in different numbers of days. It would make absolutely no economical sense to kill the plant before it ripened.
.
I'm a farmer and we raise cereal crops (among other things). The process of maturity isn't quite as straightforward as is described above.
Cereals (all cultivated crops, for that matter) reach a stage of development where the kernels are fully formed but the moisture content of the grain is too high for safe, long term storage. Maturity dates vary by specific variety (there are thousnds of varieties of wheat and barley), but most reach maturity in +/- 100 days after planting. Further drying, to reach the moisture content at which the grain can be safely stored, will usually take an additional week or two.
Most regions where cereals are widely grown have growing seasons long enough to provide the necessary number of frost free days for the crops to dry naturally. In some, more northern, regions, cereals are desiccated to reach the desired moisture content before a killing frost can damage the not-quite-dry kernels. In other areas, cereal crops may be desiccated to expedite harvest so that a subsequent crop can be planted a bit earlier (regions with long growing seasons, for example).
Since most cereals (corn being a notable exception) have relatively short growing seasons, desiccating isn't necessary. There are enough places where cereals can be grown, however, which have different climatic or agronomic situations, where cereals are desiccated. The fact that glyphosate is cheap and available worldwide is a factor when a grower is considering whether or not to dessicate.
We have never found it necessary to dessicate cereals. It's not a common practice, but it is done in some parts of the world. Whether it is a safe or desireable practice is a matter of opinion which won't be resolved on a homebrew forum.
Mark