Sometimes people use "here/there" sometimes "over here/there" what is the difference?
My personal sentiment is that much of the difference is merely cultural, such as the difference between you guys, you all, and y’all among American English speakers when addressing a group of people.
However, there are some times where they are not interchangeable. It's very hard to make rules for these examples because there are parts to the rules.
Specifically, when you use the word over you are referring to distance.
In one example, I say: “Matt, come here please.” The message I want to convey is that I would like Matt to be at the same location as me. I don’t want to communicate anything about the distance or area traveled to get to me. The emphasis is him coming to where I am.
However, in another example, I say: “Matt, come over here please.” The message I am conveying emphasizes his need to travel a distance to get to my location. The distance can be short or long. I am communicating that I realize there is an effort to be made, an area to be covered, or a distance to be traveled.
An example using there, I say: “Do you see that ball over there?” My emphasis is on the distance between us and the ball which partially communicates to the other person that they may have to look far away. I would not say, “Do you see that ball there?” It would be understood but it would not sound right.
Another example, I say: “Spain is beautiful. I’ve been there many times.” My emphasis is not the distance traveled or the travel itself. My emphasis is Spain. It makes more sense to use only the word there, not over there. I would not say, “I’ve been over there many times.” It would be ok in this case but it sounds a little off.
The inclusion of 'over' as a premodifier to the locatives / directionals 'here' or 'there' connotes the intervening area. It can be as small an area as [across] the table / road (regarded as the 2-D footprint) / room , or as large an area as the English Channel / Atlantic .
The expression 'over there' was famously used in a 1910s song:
"Over There" is a 1917 song popular with United States soldiers in both world wars. It was a propaganda song designed to galvanize American young men to enlist in the army and fight the "Hun". [ Wikipedia ]
This corresponds to the adverbial usage given by ODO:
1 Expressing passage or trajectory across a [possibly unspecified but deducible] area:
he leant over and tapped me on the hand