They serve to indicate the scroll position and the relative size of the scrolled view (and the fact that there is a scrollable view in the first place), and to quickly scroll a longer range.
They still exist (they appear when scrolling), but for regular use with a trackpad you don’t need to move your pointer into the scrollbar. Having them always visible is unnecessary and gets crowded when you have more than one scrollable region.
It is annoying to to have to perform a UI action to see the scrollbar (or to see if there is a scrollbar) instead of it just being there. Also, many people work with a regular keyboard with mouse, and may be operating mostly with the keyboard, and now have to reach for the mouse just to see the scrollbar. The scrollbar is an important UI element even if you just use PgUp/PgDn etc. to scroll by keyboard.
You might as well argue that it is not necessary to have the status bar always visible, or any other type of indicator.
I also don’t buy the “crowded” argument, because scrollbars were perfectly fine when we only had 14" monitors with 1024x768 resolution.
There are 3 settings on macs: Always show scrollbars, only show when scrolling, and show scrollbars if a mouse is connected. Set it to the one you like. I can go without scrollbars and you can have scrollbars displayed.