A website should work properly on phones, tablets, and desktops, not just look good on one screen size.
Your site may be viewed from:
phones
tablets
laptops
desktops
different browsers
different screen sizes
That means the site needs to adapt properly.
A responsive website adjusts its layout and presentation depending on the screen size.
This may affect:
spacing
navigation
text size
image scaling
button placement
section stacking
readability
If a mobile user lands on a site that is hard to read or use, they are far more likely to leave.
A responsive website improves:
usability
readability
accessibility
professionalism
conversion potential
A website that is not properly responsive creates friction where you can least afford it. Good responsiveness makes the experience smoother and more practical for real users.