Geographic

The first obvious modifier is geographic. If your products or services are limited by geographic area, it is probably not worth going after a highly competitive term that does not specify a region. For example, a homebuilder who worked exclusively in the Atlanta area would probably not find it worth his time or effort to target the general keyphrase "homebuilders", considering that good positions for one word keyphrases are exceedingly difficult to obtain and that the vast majority of people who type this keyphrase are not looking for a homebuilder in the Atlanta area. Adding geographic modifiers (in this case "Atlanta homebuilders" or "homebuilders in Atlanta") makes search engine keyphrases easier to target and also attracts a much more targeted visitor.

Descriptive (adjective)

If you only sell blue widgets, and there are many other colors of widgets available to the public, a large percentage of people searching for "widgets" are probably not your target audience. As with geographic modifiers, the addition of descriptive adjectives makes your search engine keyphrases easier to target ("blue widgets" will almost always be easier to target than "widgets" alone). Also like geographic modifiers, descriptive adjectives help attract visitors who are looking for exactly the products or services that you offer. Why should you expend huge amounts of effort to achieve high rankings for a phrase when the traffic from it isn't comprised primarily of your ideal visitors?

Descriptive (noun)

The addition of a descriptive noun can take a keyphrase that attracts diverse, non targeted traffic and transform it into a phrase that attracts exactly the type of traffic that you seek. Assume, for example, that you owned a company that specialized in internet marketing. When you consider the keyphrase "internet marketing", it's easy to see that a person searching for that phrase can have many motivations - not the least of which would be trying to learn how to do it themselves. When you add a descriptive noun modifier, such as "companies", "consultants", or "firms", you are suddenly targeting exactly the type of traffic you want - someone who is at the point where they are looking for a company that offers the service (not information on the service itself). Of course, as with all other modifiers, this also has the additional benefit of making the keyphrase much easier to target.

Low Quality Modifiers

The likely motivation of the searcher always comes into play when you are trying to decide upon keyphrases, and there are many modifiers which generally (but not always) attract the wrong type of traffic. These include "fr'ee", "sample", "ideas", "advice", etc. Let's assume you have a company that specializes in email marketing. Adding low quality modifiers would give you terms such as "email marketing advice", "email marketing ideas", "fr'ee email marketing", etc. These terms would probably attract what can be called "conversion traffic". Are there a handful of these people, clearly looking to do things on their own, that might convert to paying customers? Certainly. Should you make an effort to attract this traffic when you have better terms, such as "email marketing consultants" or "email marketing firms", to target? Probably not.

Conclusion

Modifiers in search engine phrases are used for two main reasons: to increase the percentage of ideal prospects in the traffic the phrase attracts, and to find terms that are easier to target on search engines. Adding the right type of modifiers to your search engine keyphrases will increase your chances of success. Good luck!