What if we do not use MAX function

I was supposed to use a script to get the MAX value from a table. It should have looked like this

BEGIN
DECLARE @MaxID AS INT
SELECT @MaxID = MAX([Id]) FROM dbo.suggestion
Insert into suggestion
values(@MaxID+ 1, 'value', 1);
END

Instead, due to some error, it went like this

BEGIN
DECLARE @MaxID AS INT
SELECT @MaxID = [Id] FROM dbo.suggestion
Insert into suggestion
values(@MaxID+ 1, 'value', 1);
END

It was working fine till date, due to some other issue when we started drilling down, then it came to light. I wonder how it worked till date. If somebody share their knowledge, it would be highly appreciated TIA