Smooth lines represent the change of the sound of that note from high to low. Smooth lines are used on multiple (usually more than two) notes with the same or different pitches, indicating that you should sing (play) smoothly.
when the smooth line is used in specific singing (playing), the singing (playing) effect of vocal music and instrumental music will not be much different, because the same symbol is used in different scores and has the same purpose. Moreover, human voice is the most beautiful, and the performance of musical instruments is largely imitating human voice. In this way, on the contrary, when you look at it and listen to it, you don't feel too much difference.