The difference is the means by which he goes about the task.

He quickly set about the task of building a new church.

He did not go about his task as a teacher.

After an hour had passed, he started thinking about his new task.

With a firm goal in mind, I set about my task.

The next day, when Tom set about his tasks, he found that the jobs were already doing themselves.

He turned the lights on, moved out to the kitchen and set about the task.

At the assembly area I told the companies in a few words about their new task.

Now he set about his task with the confidence of someone much older.

I didn't even want to think about the sorry task of breaking the news.