I have worked with partners my entire business career. For me, it has always been preferable to share the journey than to direct affairs as a sole trader. Perhaps I have made less money than I might have but I believe the experience has been more fun, and I've got involved in far more projects than I could have working alone.
A partner helps relieve the loneliness of being the boss and the stress of judging risks. While firms cannot function as democracies, pure dictatorships are vulnerable to all the whims and failings of an individual personality: overreach, emotions, limitations and favouritism.
The easiest partnership is a 50/50 deal, each contributing the same capital and effort, and starting at the same point in the life of an enterprise. But such arrangements may well not be feasible or fair. Often one party has more cash or time or know- how, or has already initiated the operation.