The Holy Ash is very closely associated with Lord shiva. As the God appeared as the column of fire, It was adorned by the Ash from that fire. Even today for the shiva lingam which resembles the infinite column of flame (the Lord), is anointed with this Holy Ash in specific that symbolizes this associa that symbolizes this association. Having closely associated with the Lord the Holy Ash is of very great significance for the shaivites. Here are a few samples from that great glory.
Making and Glory of Holy Ash from periya purANam
The application of Holy Ash (bhasma dhAraNa vidhi)