I see from the internet that the Queen has appointed Prince Philip to the post of Lord High Admiral to mark his 90th birthday. This is not only an appropriate honour, and indeed one of the few open to Her Majesty - what do you bestow upon someone who is KG, KT, OM etc ? - but also makes historical sense.
Since 1964 the position of Lord High Admiral has been held by the Queen; prior to that it had been in commission since the tenure of the Duke of Clarence in 1827-8 before he became King William IV. In that case it had been called out of commission, and the previous holder was in 1709.
The flag of the Lord High Admiral
There is more about the history of flag here.
Such great officerships should surely be held by someone other than the Sovereign, whom they are designed to serve. If an office reverts to the Crown then it becomes dormant until it is bestowed again. In earlier centuries monarchs were often anxious to secure such positions from hereditary holders who might (and sometimes were) a challenge. Hence the Lord High Constable of England is appointed for the Coronation and holds the post until death, when it again becomes vacant.
Keeping such posts within the royal house made perfect sense then. Today it is a ceremonial honour, and very fittingly bestowed in this instance.