well I can see what you are talking about, for example Abraham would build an altar wherever the Lord appeared or spoke to him and those places became significant in Israeli history like Hebron and Bethel, also another town which became the capital if Israel when it parted from Judah. there is also a statement by the Lord God in reference to wherever He leaves or places His Name can't remember where He said it though. building pillars was also used as a marker. Joshua placed a pillar of twelve stones in the middle of the Jorden when they crossed (wonder if its still there) I think Jacob did also don't remember where. so it is very possible that the correct altar in the correct place is where, so on and so forth with the Lord God of Israel. if you remember when Joseph's descendants where given land on the other side of the Jorden before crossing it, there was concern some time later about where their sacrifices where to be offered, and the tribes that crossed were ready to go to war over it, because the law stipulated that offerings were to be where ever the tabernacle was, unless under extreme cases, if my memory serves.
First altar with significance, was when Noah exited the Ark, where he sacrificed clean animals, only Noah and Family, After the Earth began to repopulate then men probably began to do sacrifice to deities using some sort of altar, so from Noah to Abram there were no accounts of altars being set up to offer sacrifice to God! we can only speculate about the descendants of Noah using altar's for other reason's.
God makes a promise to Abram about the Land, and Abram set's an altar up at Shechem, no mention of a sacrifice, probably a altar as a Memorial, a marker where God appeared to Him.
Then at Bethel, another alter, no sacrifice mentioned, But Abram call's on the name of the Lord! then later return's to Bethel and again call's on the name of the Lord, then on to Mamre and another alter, no sacrifice mentioned.
Then God ask's Abram to place Issac on the altar, sacrifice intended, But where was this place? the land called Moriah, which today they say it is the higher peak above the Temple Mount, Mount Mor-i-ah!!
Issac builds an alter and called on the name of the Lord, again no sacrifice mentioned, that was at Shebah, then called Be er she ba!
Jacob build's an altar, and names this place, El Elohe Israel, an altar for a Memorial. Jacob build's another back in Bethel and renames it El bethel, because God appeared to Him when he fled from Essau. Monuments and or Memorial's seem to be the significance of these altar's.
Moses builds an altar after the war with Amalek, and names the altar-" The Lord Is My Banner" again no sacrifice. Exodus 20:24 now an altar is built for sacrifice, made of un-hewn stone, after the Giving of the Ten Commandment's.
In the OP you ask why an altar, There had to be an understanding about altar's from Noah, transmitted through the decedent's, like Shem, Shem was the teacher that lived into Abraham's time. But there is also practices that were used in the Ancient day's that were profound act's that held strong implication's. Like a blood Covenant, which was the most Serious Oath one could undertake between two person's, or Tribe's.
At this point I have no real resolve as to why, only speculation, or just my opinion. Good Thread, makes the Gray Matter churn.