"Work for" vs "work in"

[Lenakul]

Hallo everybody! I need your help...

We can use different prepositions with the verb "work". In the Market Leader Course Book (Elementary level, 2004, I've found the following explanation:

Use "work for" when you talk about the company or organisation where someone is employed
Use "work as" + job
Use "work in" + kind of activity
Also, use "work in" with words like bank, hotel, hospital or factory

Everything seemed to be OK, but I became puzzled while trying to do exercises...
For example, why I cannot use FOR instead of IN in the sentence:
At the moment, he is working IN a travel agency

And why I cannot use IN instead of FOR in the sentence:
Are you going to work FOR a clothing company in Edinburgh?

Thank you!

[Mister Micawber]

.
Your course book explanation is a guideline, not a prescription.

At the moment, he is working IN/FOR a travel agency.
Are you going to work IN/FOR a clothing company in Edinburgh?

These are all OK. Another, simpler guideline: Use 'for' for the organization and 'in' for the place.
.
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Native English teacher at Mister Micawber's

[Lenakul]

Thank you very much, that's just what I needed to understand
I wonder how I couldn't guess it before, because it is really very simple

[Lenakul]

I've just thought about something else. Is there any initial reason of existing these two variants when speaking about somebody's job? Does a speaker imply anything when he says: "Helen works FOR this organisation" (for example, that Helen is not a director of this organisation)?

[Milanya]

It does not matter whether Helen is a director or not if she is not, the owner of the company. One can work for an employer and work at/in a company.
I work at a Fire Department, I work for the City of Cityname.
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con·text - The part of a text or statement that surrounds a particular word or passage and determines its meaning.