Visa Invitation for Russia is required every time that you apply for a Russian visa.
A Visa invitation is always required, regardless of whether you ask for a business visa or tourist, visitor, training or work..
A Visa invitation is the is the main and only reason for the visa obtaining.
Missing an invitation will result a denial of a visa, even if all other documents are in order.
Your Visa invitation confirms your right to apply for a Russian visa and your right to get it.
Your Visa invitation states your purpose of visit, visa validity and number of entries.