Very few system fonts are any good. Would you use Arial instead of Helvetica Neue? I certainly wouldn't. Put two posters side-by-side and you'd notice the Helvetica one as looking more professional, even without any design background.

Additionally, very few system fonts include all the weights. Fonts aren't just come in a single weight. The font you use for a giant page-filling title is generally skinnier than the font used for a caption.

Good design creates a reaction, such as causing you to buy something or interacting more with something or whatever, even for people that say they don't care about design.

Designers know you better than you know yourself.