SquareSpace is a website builder that makes it easy(er) for non-technical people to create good looking websites that work on all device sizes. SquareSpace has been a key player in the democratization of web development. SquareSpace is driven by templates — so the template you select for your website is pivotal to the final functionality included in your site. With custom code, you can overcome some template limitations, but it's best to not diverge too far from your template.
SquareSpace is the recommended tool when non-technical people want to create and manage their own website. SquareSpace puts up firm guardrails to help prevent people from drawing too far outside the lines. It's a great option when you want a good looking website that will evolve over time - and you're not comfortable learning a tool like Webflow that gives you full design flexibility.
As mentioned above, SquareSpace makes it easy(er) to create a good looking website. They have great templates that are suitable for most websites. Compared to many of the other "easy website builders" (Wix, et al), SquareSpace stands out from the crowd.
The thing that makes SquareSpace great, is also the thing that makes SquareSpace frustrating and limiting. The issues come in when you want to do something that doesn't align with your template. Because the platform isn't built to cater to full customization, even making simple changes can be a headache. Getting a bespoke look is really challenging. If you want a website that aligns closely with your brand or doesn't look like every other SquareSpace website - you'll probably need to look elsewhere.