As someone said before, the reason for the "sans-serif" fonts, is almost as old as the web itself.
First of all, the "serif" fonts (times new roman, georgia, every font with those little decorations at the end of every character ending, etc.) works well in printings, becasue of the size, the space, the junctions between characters, etc. But it would never work well when you make it small. In the other hand, the "sans-serif" fonts work well in both scenarios, just controlling the character width, the separation between characters and you don't have visibility issues in small sizes let's say 10px, 9px, even 8px, where you can still read and not get bothered by the decorations of the "serif" fonts that in those sizes start to make a mess.
Second, the CSS declaration for the font-family, has another reasons as I recall. The first font is the one you would like to use let's say "Segoe UI". It's a good font, has good size, looks great small, etc. But is not that common right now, so I should choose another font that works just as well and in case not being the first one the browser should use this instead. And so on. The real problem is that you must acknowledge that every font have some different outter spaces, space between characters, space between words, so even being from the same family sans-serif they are not exactly the same.
Helvetica is a very famous font because all what I said before: looks great big, small, very small, it's wide, narrow, tall, short, etc. its proportions let it be that way, but is not as commonly used in every computer because its proprietary, and if you got it, then you use a Mac or you bought it (unless it has come with a graphic program and then is just one type of Helvetica).
So the statement should be:
font-family: helvetica, arial, sans-serif;
You shouldn't use variable width character fonts with fixed width character fonts in the same statement, because monospaced (same space between characters) and variable width don't look even close each other, but all that requires more knowledge when to use it and where to apply.
I myself use these:
font-family: calibri, arial, sans-serif;
font-family: 'Segoe UI', arial, sans-serif;
Calibri is very very small but is easy to read, and Segoe is kind of Verdana but the elegant version.
In any case, Arial and Helvetica are almost the same.
See ya.
