This is quite a complex area. By default, YouTube videos are NOT licensed for use in church but certain copyright owners are still happy for their content to be used in that way. As a rule of thumb, CCLI recommend that you get written permission from each original copyright holder before using any of their videos in church.
iSingWorship are produced especially for use in churches so there is no need to worry about additional licensing.
Most YouTube backing tracks are recorded with the original artist's song key in mind and those are rarely useful for a congregation to sing along with. For example, if the original is a male singer, you may find that female singers in your congregation are struggling to reach the higher notes in the song.
Many worship songs found on YouTube contain extended instrumental breaks and additional repeated sections. All the song arrangements used in iSingWorship are produced as if they were a live band leading in church without too many repeats or extended instrumentals.
Songs found on YouTube have come from multiple different sources, perhaps from a company, a hobbyist at home or a local church. All songs produced for iSingWorship have been mastered to the same level specification meaning that whichever style of songs you choose for your service, they will all be playing at a consistent volume.
Using YouTube videos takes a lot of extra time to research whether it looks suitable for congregational use. Many contain lyrics that are too small for some to read, have lyrics that change too slowly, or have busy and distracting backgrounds. All songs are carefully designed with consistently clear and legible lyrics, soft backgrounds that don't become distracting in worship and the unique 'next line' feature which makes singing along easier and more natural than ever before.
iSingWorship gives you the ability to customise songs in all kinds of ways. You can repeat or miss out song sections, change the key and/or tempo, change the lyrics, play different arrangements of the song, play softer mixes with no drums and electric guitars, play unplugged versions and much more. When using YouTube videos, you are tied to the style, length and key of the recording on the video itself.
Most importantly and as mentioned at the top of this page, using iSingWorship means you don't have to seek permission from content owners to use them in your church or small groups.