After completion of the wudhu of two Rakaats is full of blessings (sawaab). It is related in the Hadith that performance of the two Rakaats of Tahiyatul wudhu Namaaz makes paradise obligatory (wajib) for the performer (ref:Muslim Sharif) There are also many other benefits.
It should not be performed during the Improper (makruh) times. (When the sun rises, when it is at Zenith and when it sets.)
After the Fajar Namaaz one should sit at the same place and recite Durud Sharif, Zikr, tasbeh., Quran or listen to religious talks, etc. without participation in worldly affairs until the sun has fully risen, then perform two or four Rakaats. This is called Ishraak Namaaz.
Performance of this Namaaz brings blessings (sawab) equal to a Haj or Umrah. (Ref. Tirmizi Sharif ). Another Tradition says performance of this Namaaz(Prayer) wipes away all ones sins. (Ref: Abu Dawood). One can perform the Nammaz even after one has indulged in worldly affairs although the sawab will be much less.