There is likely to be a number of reasons. For a start, they are a great item to hand out for prizes and competitions. For those not affiliated directly with companies (i.e. those not employed by them), they are a piece of clothing that is far more likely to be used – whether in public or just around the house – when compared with other types of clothing, which may be seasonal or specific situations.
Because polo shirts are flexible, comfortable and lightweight, they can also be selected as workwear for the staff, for them to wear while they work. In an industry in which the suit is too restrictive and a little too professional, but something like a typical t-shirt you will not be exact, polyester polo shirts might be ideal in between. For example, full-on suit may be too uncomfortable and too hot for people in the trade operations – as builders, electricians, and so on – given the style and type of work they do. As well as displaying a company logo, uniform use of the promotion will help people to make sure that professionals and / or members of staff in certain scenarios.
They are usually a setting in which it is important for employees to act both professionally and friendly, where a suit can appear a tad too full-on and may actually run the risk of customers delaying possible.
Of course, there may be industries in which this kind of polo shirt unprofessional. The lawsuit could potentially still clothing choice for those in the finance industry and lawyers, for example. Keep in mind that when it comes to advertising a company, it is very important to know what is acceptable and what may be more harm than good to the image of the company and the way they sell their products.