Sure it’s cheaper ( or even free) to get friends or relatives to play, but by choosing to go this route you may experience some bumps, detours, or roadblocks you may not have anticipated.
A written contract between you and the musician you hire spells out the obligations of both parties. The performer is required to fulfill the terms of the contract. You won’t be at the mercy of the musician bailing on you due to stage fright, lack of preparation, showing up late, or not at all.
An amateur or inexperienced player often under-estimates how much music is required. It’s better to hear 40 different pieces of music in a set list than the same 10 pieces repeated 4 times.
An experienced performer will have more than the required amount of music in case the client wants the musician to play longer or if the event is delayed at the outset. (At the first wedding I ever played for, the bride and bridesmaids were almost an hour late.)
An experienced musician will ask you what type of atmosphere you want to create for your event. Based on your feedback, the musician will offer suggestions from their extensive repertoire as to what music will best capture the theme and mood of your event, and be open to any special requests. An amateur performer may not have the repertoire to accommodate this.
In the case of wedding processional music, an experienced performer will know how to best time the music to finish when the bridal party completes the procession.
A professional musician will have a portable piano in the event the venue does not have one and amplifiers should the size of the room and number of guests warrant it. Make sure your friend or relative has the necessary equipment or you may be on the hook to find a suitable instrument and necessary amplification.
Best wishes for a successful and memorable event!