In my case usually is do a change and score a goal xD I have usually good experiences after do a substitution.
If the match is so close for example, I don't like to do sub's because my better players are in the 11 and I prefeer to assure the control... but if I have the control in possession terms, and I have the sensation that I can have more, I just change the player MC/ST that I don't see in the comments during 50 minutes.
If, is close and recive a goal of course, too do a sub' but always in 2nd half and for the players that don't appear, or, fail continously when try to shoot, maybe too in some matches I see a player more aggressive than usual, and when have a yellow too I do the sub'... but 1st is to have the feeling of control of the match.