What is the difference between:
I have to know exactly why it is I'm doing a particular job.
I have to know exactly why it is I'm doing a job.
They mean almost the exact same thing.
How is that different from "They mean almost the same thing."?
And the answer is... that it's not so different.
Let's say "task" instead of "job". So, every task is probably different. "What about this task" versus "What about this particular task"? The phrasing is distinguishing more strongly that each task is different from the others. They have special characteristics which differ from the general characteristics. You are emphasizing the point that jobs are each special.
Another reason to include a word like "exact" or "particular" is for it's literary quality or the way it sounds when spoken. The words add a certain style. It's similar to saying "very", which creates an emphasis, even when that is not always required.
The word 'particular' isolates the job I'm talking about from other jobs. If you ask
Why am I doing this job?
the answer is likely to be the same as for any job. Because I need to earn a living or because somebody asked me to or because it's on my list of jobs to do.
But if you use 'particular'
Why am I doing this particular job?
the answer is more likely to be specific to that job, as in 'Why am I doing this job and not a different job?' Because this job needs to be done first or because I like this job better than other jobs I could be doing or because I can earn more money doing this job than a different one.