There are several types of product research that you'll need. First, you'll need to search patents to make sure that your product doesn't infringe on any existing patents. If you find that it doesn't, you'll need help submitting your own patent drawings. This can be a confusing process for the amateur, and as such it's usually best to choose a company to help you through it.
You'll also want to do product research to make sure that there's an interest for your idea. The last thing you want is to get all the way through the prototyping, manufacturing and development of your product, only to find that people simply don't get your idea, or aren't willing to pay for it. Hiring a company to do market research to find out if there's an interest in your product can save you a lot of future heartbreak.
When doing product research within the market, it's almost always best to have someone else, other than yourself, handle it, for a few reasons. First of all, it's sometimes difficult for the creator of a product to listen to feedback from consumers without becoming defensive. In most cases, it's best to have an unbiased third party doing the research so that they're able to assess the feedback they're getting and figure out how to include it in further development.
Another issue with doing your own product research through the market is that you simply won't have the experience. It's common for folks who don't know what they're doing to accidentally ask leading questions, or not to not even know what types of things to poll people on. When you hire an experienced company, on the other hand, they will know what to ask and how to ask it, to get the information they need.
Of course there are many other kinds of product research as well. You'll need to research the market to find out what the competitors are and to find out how your product can be created and marketed more effectively. You'll also need to do safety testing, and testing to ensure that your product lives up to the claims that you'll make about it.
As you can see, it makes sense to hire an experienced company to help with product research. They'll start by making sure that you're not dealing with a previously patented product. Other types of testing they'll do will depend largely on what type of product you're dealing with and how your target audience is. No matter what you're trying to sell, it's best to turn to the experts.