Your eyes can dilate behind sunglasses, especially when the main source of light (the sun) is blocked, and ambient light decreases. Regular sunglasses offer some protection against certain UV wavelengths, but not enough to protect your eyes when looking directly at the sun (or, during an eclipse, the corona). Most sunglasses are mainly designed to, first and foremost, look good. After that, there is some protection against UVA (400-315 nm) and UVB (280-315 nm) light. Some sunglasses also offer "UV400" protection, which is short-wavelength blue light longer than 400 nm. Polarized sunglasses do not protect against UV at all, only reflected light (such as shining off the water or from an automobile's body or glass) and glare. Different sunglasses tints (or colors) also do not protect against UV, but they can affect color perception.
Surely wearing proper sunglasses at any time is safer than not?
That is true, it is safer, the question is to what degree? Sunglasses offer some protection from indirect UV and visible light that has been scattered by the atmosphere and reflected off of things around us. However, the number and energy of photons (the "particles" of light) reaching your eyes when looking directly at the sun is dramatically greater, and so much more dangerous. Eyes can be damaged quite quickly by high-energy light of many wavelengths, including UV and visible, with the magnitude of the damage correlating to the energy of the light. Damage is also cumulative, accruing over a lifetime. Even small peeks directly at the sun will contribute to your overall lifetime risk of eye damage, and may cause permanent, irreversible harm very quickly.
There are a number of much safer alternatives for watching solar eclipses, including pinhole projectors, welding goggles rated higher than 14, various solar filters for cameras, telescopes, and binoculars, and fully exposed and developed black-and-white film (good luck finding that nowadays), among others.
Above all, make sure you're safe, and if you have any doubts consult with an expert, such as a science/natural history museum or local university's astronomy department.