Logos are everywhere, and we see countless ads every day for services of all kinds. Because logos are so prevalent, buyers do have a general idea of what a good or bad logo looks like and I'd bet they'll know whether you designed yours yourself, or hired a professional.
Your logo, or lack of one, gives buyers an immediate first impression, which will determine what their snap-judgement is of the business as a whole. Like it or not, most customers do (subconsciously) consider the company logo before they decide whether or not to trust it. Your logo has the opportunity to make a positive or negative impact on the buyer, so take advantage of that!
If you've never had a logo designed, and your company "logo" consists of something like a line (or more) of text in the Papyrus font for example, a lot of your customers notice that lack of detail and will likely wonder why that major part of your company identity apparently isn't important to you. They may even wonder what other important details of your business are left unattended to! Uh-oh!
The other problem with a non-logo, or lack of one, is that it makes your business extremely forgettable, especially to potential customers. Once the customer has come inside (if you have a physical business location) and made some form of positive contact with your employees, that customer may find it harder to forget the business.
If the business was seen and immediately forgotten by a potential customer (due to the lack of a professional, memorable business brand) it's an opportunity lost, because that customer may never come inside to learn that they could've had a positive experience there.
Without some sort of branding to make your business stand out, it could be easy to forget. If you're a small business, but have the professional branding of a corporation, your customers (and potential customers) will notice and appreciate the level of stability and professionalism that type of attention to detail brings to a company. And they will likely put their trust in you, over your more professional competition.
What if you're doing really well already, but you don't have any kind of serious branding? Well, you've been lucky so far. But you don't want the success of your business to always be based on luck, do you? Take that extra step to ensure your success!
Whether you're aware of it or not, there are probably some products or services that you buy because you're just plain loyal to the brand. Either you like the product so much you aren't ready to investigate other versions, or you like the company so much you don't want to send your business elsewhere.
You can give your own customers that opportunity when you create such a significant and successful brand for your own business. And as we all know, brand loyalty can keep your customers coming back for years!