Is clause necessary?

Whether "while she types them" is necessary depends on the meaning you are trying to convey.

Usually what happens is that a "boss" [what century are you living in?] dictates letters to a secretary who writes them down in shorthand and types them later. So in that situation you might write that the "boss" dictates a number of letters each day to his secretary. The stuff about typing etc can be left unstated.

If however, you wish to emphasise the remarkable and unusual method of working that a particular team of "boss" and secretary have, where that secretary types out the letters at the same time as the boss dictates them, then maybe you would feel like mentioning it. [Can't the "boss" use a word processor?)

In general, the writer must decide whether to leave out such details or whether to include them.

Do they still have such old fashioned working practices where you live?

Thanks, Contrex, for your detailed reply.

Do you perhaps mean "The boss dictates a number of letters to his secretary every day, and she types them." While "while" can indicate a contrast between two things (as I am using it in this sentence, for example), it is more commonly read as indicating simultaneity between two things, as I read it in your sentence in question, and as contrex read it in his reply. So, while technically your sentence may be correct, it is probably not going to convey to your readers what you're probably trying to convey to them. Unless, as contrex points out, they have a truly peculiar working method (and are seemingly stuck in the 1950s in terms of working methods).

Thanks, Username.

I agree with your comments.

Yes. Whereas the use of "while" to show contrast is common and accepted in informal usage, in formal writing, "while" should be reserved to show that two or more events occur at the same time. For comparisons and contrasts, "whereas" and "although" should be used instead of "while."

Indiscriminate use of "while" can result in unintentional ambiguity, of the kind found in one variety of what used to be called the "schoolboy howler".

* While my cat eats quickly, she enjoys a leisurely stroll.
* While Charles I was beheaded, Charles II lived to a ripe old age.