To answer the first part of the question is easy:
Architects (registered and chartered as such in the UK) are the only building design professionals qualified as such and allowed to use the job title other than golf course architects, landscape architects or naval architects under British Law.
Being chartered we must comply with two separate codes of professional conduct and be fully insured and subject to two separate independent disciplinary procedures as well. It is therefore not only very unlikely that an architect will perform in an unprofessional way as we can be 'struck off'.
That is not the case with those offering 'architectural services' who are not architects or chartered professionals. Also only architects are trained to above degree level in Architecture despite the number of job titles with 'architectural' in front of them.
Answering the second is more obvious to us than perhaps it is to those outside the profession:
By using an architect we use our knowledge, skills, training and experience to add more value to any building project than that added by any non architect (with the exception of those who could qualify but choose not to, thus depriving you their client of much legal protection). By knowing how proportion, colour and volume affect aesthetics and how materials affect cost and performance (thermal, structural and acoustic etc) we, for a large project, incorporate and orchestrate the design input from other chartered professionals to best meet your needs, not our egos.
To do that, as a rule, many of us no longer use drawing boards or CAD but only the latest Building Information Modelling (BIM) software in order to 'build' a virtual model of the project to check for clashes between the various design inputs, ensure any errors are corrected and prepare a full set of construction information for the contractor(s) to build and independently administer the building contract certifying progress, payment and completion.
None of our skills or knowledge etc can be gained through experience alone no matter what those who don't know might tell you.
As an architect's skills are best utilised from the earliest stage of a project please do contact me, even if you are only at the 'business case' or 'finding a site' stage. The earlier we are involved the more value we can add.
01536 522586 / firstname.lastname@example.org