Why are aunts and uncles referred to by both their relation to you, as well as their name?

You can have more than one uncle or aunt so you use the name and relational title to identify the specific person.  My elder niece and nephew addressed their maternal and maternal grandmothers both as 'nanna' but referred to them by their given names. My younger niece and her brothers use different relational titles to refer to and address their grandfathers (grandad on mother’s side poppy on fathers side) so they don’t say the given or surname of the person they’re talking to or about. 

I personally don’t expect my nieces and nephews to use the word uncle when referring to me.  

There may be other relatives that share their name, thus cutting down on the confusion of who you are referring to.

My grandparents were referred to as Grandma or Granddad Surname

but Aunts and Uncles were by by given name only. 

Great Aunts and uncles were also referred to in the same way as aunts.

It depends on the family.  In my Family Grandparents were called by title as well as last name.   Aunts and Uncles by their title and first name.

I have two nephews.  Both just use Tom (just as glad not to be known as an Uncle Tom). They have four other living uncles, but rarely talk to most of them.  But the younger one, when he was little (<6) called me Unka Tommy.

My nephews and nieces are all aged between 21 and 41, I’m sure none of them call me Uncle ....

Because grandparents aren't together all that often, but you may have several aunts and uncles present at the same time. Most parents only have one mother and father but may have multiple siblings.

Nothing at all to do with genealogy research........

It comes from the historical tradition of respecting your elders

As a mark of respect for elders and / or a way of distinguishing family members with similar names?

For example, grandparents are given most respect with 'grandad surname'. aunts and uncles being one generation above get some respect, so 'auntie forename'. Cousins and siblings are equals, so forename only but sometimes preceded with 'cousin', bringing us to the distinction hypothesis:

My father's brother is John, who has a son John, while I also have a brother John. Adding the relationship into the name distinguishes who is who (from your point of view) Uncle John is my father's brother, cousin John is my uncles son, our John is my brother.

They aren't, in all families and all countries.  

In many families people only use their uncles' and aunts' first names. 

My nephew and niece just call me by my first name.

My mother always used to refer to her cousin as 'Cousin Mary'.

When there are two sets of grandparents, children often refer to one as 'Granny Sue' or whatever, and the other as 'Granny May'.

In the north of England, people refer to close family member with the prefix 'Our' - so their brother would be called 'Our John', etc.