Outgoing, confident people can find a supportive social network with people like themselves anywhere. Introverted and/or insecure people need a structure that functions around them and allows them to integrate slowly and passively fit in somewhere at first.
The irony of the church's totally non-professional local ministry structure is that the outgoing, confident people sometimes overwhelm the introverted and the insecure. It's a bit counter-intuitive, but the wards and branches that work the best generally are the ones that have leaders who are acutely aware of their shortcomings - the ones who often feel like they are on the verge of sinking every time they jump in the pool. We had a Relief Society President, for example, who was extraordinary - but one of the reasons she was so wonderful was that she knew she was nothing of herself. Her calling scared her nearly to death, but she accepted it, anyway.
Personally, I think that's the main reason why callings are shuffled regularly - to avoid the natural arrogance that arises when people finally feel like they "get it" and can perform their calling in their sleep. Not only does that provide opportunities for "unnatural" growth to the truly meek and humble who normally wouldn't be chosen as leaders, but it shortens the harmful period that inevitably occurs for some members when there is a personality conflict with a leader whose personality is opposite of their own.
I appreciate the words of newly called apostles, since they invariably speak sincerely of being overwhelmed by their new calling.