People who are using Twitter for business purposes are searching for other people in or related to their industry. If I am an accountant, I want to use Twitter to communicate with other accountants, lawyers and business consultants – anyone who might be in position to hire or recommend an accountant.
You want people to find you – either by your tweets or the information in your profile. Since new followers are found by searching for them, you need to use the keywords that are being used to search for people or conversations related to your industry. The accountant in the above example would want Marketing Consultants to use keywords related to accounting, finance, taxes, etc., in his posts, so that they appear in searches that contain those keywords.
You can use search engine-based keywords tools to do your keyword research, under the assumption that the keywords that are used to perform searches in Google are the same as would be used to search Twitter. That may or may not be the case. So it definitely makes sense to also do Twitter-specific keyword research. Find out exactly what search terms are being used to search Twitter, and what keywords are being used in tweets, that are applicable to your business.