Most websites are intended to be used by humans.
Check whatever your site is human-compatible, at least ask your family/friends to use it and to report problem they encountered.
Check site from view point of someone that opened given page, spends 3 seconds on it and may stay longer.
For example on page with long text and image do not expect that users will read the text. It is likely that they will look at hopefully interesting image and become confused because image and explanation is disjointed.
It may be especially disappointing if image is not really image but something interactive (for example a slippy map) and user completely missed that.
It is a good idea to run automatic tests first, it is sad to get feedback from expensive[1] human testers that could be spotted by automatic tools.
[1] directly in money or in time or goodwill