TL;DR: It won't change combat much. It will make non-combat less interesting.
Obviously, it depends on the group. Are all your players trying to get the best numeric advantage, etc?
Let's assume they are (because that makes math easier) A standard 27 point buy is likely to lead to a 15, 15, 15, 8, 8, 8, or 15, 15, 13, 10, 10, 8 array (or similar). Typically DnD 5e characters have a stat they really want to boost (Str. for a non-finessing melee type, Casting stat for spellcasters, etc.) That's not going to change.
However, you have a lot less freedom in a 15 point buy. I imagine every character (again, trying to maximize effectiveness in encounters) will be 14, 12, 12, 8, 8, 8. Because the +1's in your secondary attributes (e.g. con) are worth more than avoiding a negative in a tertiary attribute, and after the 14 its all 1 point for +1 stat. And there's no point to odd stats unless you have a +1 from race. If they are playing a race where they get a +1 to the main stat, it would be 15, 12, 10, 8, 8, 8.
So, your characters will do as much damage as reliably with their main attack. They will have either a lower AC or HP/level by 1 (assuming the secondary stat is Dex or Con, which it usually is). Half of their saves will be one lower (their 2nd, 4th and 5th favorite stat). And the characters will feel a lot more similar, because where you're losing the customization is mostly felt in which stats they want to be 4th and 5th.
It also will either (a) prevent anyone from multiclassing (only one stat above 13!) or (b) make you less effective (by 1 to your modifier) while multiclassing (using a 13, 13, 13, 8, 8, 8 array) unless doing something like bard/sorcerer where the same stat is the "main" driver of the class. This 1 point will probably make the multiclass character less effective over the course of the game, but probably won't be noticed at any point in time. However, this player may notice that he misses a lot of attack roles by 1/a lot of enemies make saves by 1 and feel a upset because other players didn't get that decrease to their main stat when you decreased the stat buy.
Also, if you are playing with feats, you have to think about this discouraging people from buying the +1 stat feats. Those are great if you have an odd stat already because you get the numeric +1 to the mod a cool ability. But if you push your players to 14, 12, 12, 8, 8, 8 those feats cost a lot more.