Signal conditioning is still an important part of EE design. Not all signals are TTL level and unipolar.

For example, this appnote[0] uses a Delta-Sigma converter to digitize a load cell, with the ability to "detect one sheet of copier paper dropped on a stack of 20 reams". In order to do this, it relies on an intrumentation amplifier and custom correlated-double sampling to get a measurement accurate to 200,000 counts.

And even if your input signal can be directly input to your ADC, you'll still need an analog anti-aliasing filter.

Signal conditioning is an important part of signal processing that has to happen in the analog domain.

[0] http://cds.linear.com/docs/en/application-note/an96fa.pdf