Few jobs will allow you to implement the unique combination of strengths that only you have. If you have very contrasting work needs it will be well nigh impossible to find a single job that will enable you to satisfy them all.
You are ultimately your own boss even if you are working for half a dozen different organisations.
You can balance your paid and non-paid work.
Some do it to gain freedom from corporate agendas and politics.
Some do it to follow multiple passions or for personal growth and fulfilment.
Some do it for the pace and constant change. Others like the excitement and the unpredictability that can accompany this workstyle.
In theory there can be more leisure time but in practice many portfolio careerists say that they actually get less time to play.
It can enable you to spread the risk, allowing you, for example, to earn money from one area while building up a new business or activity in another. You might wish to continue your existing career/job on a part-time, freelance or consultancy basis.