Many church members feel it is wrong to sing during the moments when the Lord's Supper is being served. The Bible does not prohibit singing during the Lord's Supper, but tradition asserts that we must have absolute silence when we are partaking of the bread and the fruit of the vine. After the loaf has been served many churches need about 60 seconds to get ready for the serving of the fruit of the vine. The song leader might fill that minute with a spontaneous hymn. Most people will not object to singing a song between the serving of the bread and the serving of the cup.
Most church members feel it is wrong to conclude a sermon without an invitation song.
Invitation songs were invented about 200 years ago and we have been enslaved to them since then. Most brethren do not care which song we sing just as long as we sing an invitation song. In fact, every song in the hymnal might be considered a possible invitation song. Widen your repertory of invitation songs and we might rejuvenate an old worn-out custom. Most churches demand that the service end with a closing song and a closing prayer.