Cover letters, or now covering emails, can sharpen your approach to recruitment agents and companies alike, whether it is in response to an advert or a speculative approach.

When responding to an agent or employer's advert, think of the time that has been put into its preparation - the identification of the vacancy, the authorisation to hire, checking and enhancing the job specification, the first draft of the advert through to its final draft (often done by a copy writer), the choice of media and placing the advert - all done before the agent or employer has seen a single candidate. The covering email must therefore do at least two things,

a) acknowledge the effort made to date by the employer/agent, and

b) promote you toward the key elements of the advert.

In the case of a speculative approach, then the cover letter should pick up on some of the special features of the Company i.e. demonstrate your interest is substantial and also promote some of your especially relevant skills and experience.
This degree of focus pays dividends when working in the 'unstructured' recruitment market, and our expert CV writers can support you.