why we can add constructers in fld types?

public RegExTextField(SPFieldCollection fields, String fieldName)
: base(fields, fieldName)
{
this.Init();
}

public RegExTextField(SPFieldCollection fields, String typeName, String displayName)
: base(fields, typeName, displayName)
{
this.Init();
}
hi,what is the purpose of these two constructers in fld types.why we can init() method in fld type constructer,what is the usage of init() method in .net programing.