Many pages build their content with scripts after loading. A plain request gets an empty shell; a browser runs the scripts and gets the real page. Tools such as Playwright, Puppeteer and Selenium start a browser and tell it what to click and read.
A browser downloads everything a page asks for, so it uses far more proxy traffic than a plain request. Blocking images, fonts and video in the browser cuts the bill sharply. Each of these tools takes the proxy and its login in a slightly different place.