To make sure you're targeting the right people for your website, you need UX research. Your marketing team might know exactly who you want to buy your product, but is your website tailored to those people and how they user the web?
With full user research, you can be confident that your website caters to the thought patterns of your ideal customers. This involves:
This data isn't what you'd find out simply by asking people. Even in a questionnaire, people write what makes them look good rather than the full truth. In order to properly find out who your website can help, a User Researcher can give you everything you need and translate that real data into requirements for your website to meet.
Generally, people don't tend to give honest answers. This is simply because they'll say something that makes them look more favourable, or is biased in some way by their previous experience. It's also fairly hard to get people to open up about questions that don't seem relevant to your website. For example, why do you need to know that their hobby is gardening, if you run a financial tech business? It's all relevant in building up a picture of how they use the internet and why they'd choose to click your website.
You can't measure these results with traditional methods. User research is all about collecting lots of data and compiling it to see common elements. Once you've got enough commonalities in the data you can start to build something around them. This means all those seemingly trivial pieces of information become useful for understanding and empathising with your users. Once you've built up empathy, you can appreciate all the pitfalls your users get into when using your website, and this will help you spot (and fix) all sorts of frustrating niggles.