gEDA-user: pintype attritubute and gnetlist

[Ostheller, Joel A.]

>From : http://geda.seul.org/wiki/geda:master_attributes_list

  pintype
  Each pin must have a pintype=value attribute attached to it and should be
make hidden. Table 1 shows valid values for   
  this attribute. his attribute is not used extensively in the symbol
library, but it will be used for DRC and 
  netlisting. xamples: pintype=clk pintype=in pintype=pas

I do not understand why this is needed. Why does PCB need to know if a Pin
on an IC is an output, input, clock, power, ground, Tristate, etc? 

I ask because when I run gnetlist, I am receiving the following two errors:

  ERROR: Pin(s) with pintype 'output': U101:14 U101:13 
          are connected by net 'unnamed_net2'
          to pin(s) with pintype 'output': U101:14 U101:13 
  ERROR: Pin(s) with pintype 'output': U101:16 U101:5 
          are connected by net 'GND'
          to pin(s) with pintype 'output': U101:16 U101:5 

And I do not fundamentally understand why I should even care about this...