Why did Yaakov set up the stone as a 'Matzevah'?
Seforno: A sign that it the matter should be firmly established ('Yatziv v'Kayam').
Malbim: The Bris obligated Lavan only not to harm Yaakov, a revealed matter, so people can be witnesses; he told Lavan's 'brothers' to gather stones, to remind them. Yaakov was also obligated also not to deny intimacy to Lavan's daughters 1 - a hidden matter. Only Hashem can be a witness about it, so he made a Matzevah (refer to 31:48:151:3).
Ha'amek Davar: It was to offer on it. A Kadosh matter is a witness to the sin of annulling the Bris. It says 'Haramah,' for he elevated it above other stones, like "v'Yarem Es ha'Machtos" (Bamidbar 17:2); like it says in Menachos 99a. 2