The purpose of a pre-application consultation is to improve the quality of a planning application, to avoid the necessity for seeking additional information where possible, and in some cases to spare the costs of what is likely to prove an unsuccessful application.
Consultations can be of value in applying development plan objectives to a particular site, and especially assessing how the design treatment responds to local context, thus allowing the planning authority to input to design and layout at an early stage. They also allow the planning authority the opportunity to suggest that specialist advice be sought if appropriate e.g. in relation to conservation issues. It also enables the PA to advise applicants of procedural requirements such as planning application requirements, applications likely to involve an EIS, compliance with government guidelines e.g. Rural Housing Guidelines, Sustainable Residential Development Guidelines etc., possible exemptions for minor development and the implications of Building Control Legislation such as fire safety and access for the disabled.
To ensure the consultation is productive, you may be required to submit a certain minimum level of documentation in advance of a pre-planning meeting e.g. a site location map, contact details, approx. size of site, description of the proposed development etc. Each local authority will have its own requirements. There is no charge involved in organising a pre-planning meeting.
The overall aim is to identify any potential issues arising from a development proposal at an early enough stage in the design process, in order to avoid needless delays and/or costs after an application has been lodged. As an architect (whoever you choose) will ultimately be the designer, it seems reasonable to me that they would want to be present at such a meeting.