Pataka is a hindi word for Flag (English) or Dhwaja (Sanskrit). This Pataka or Dhwaja is unfurled at the top of shikhar of a mandir. Pataka or Dhwaja is a very important part of dressing of a mandir. There is a shlok in this regard.
Sikharam shreershamithyaahuh garbhageham galam
Yatha mantamapam kukshirithyahuh praakaaram
Jaannjamghayoh gopuram padam ithyaahuh
This shows the importance of dhwaja in a mandir. It is also a shubhshakun to see a flying Dhwaja at shikhar of a mandir
Brass Dhwaja is used for the reason that it is permanent and looks majestic.