For perfect tenses we use have as the helping verbs.

Present perfect - have/has + past participle
Past perfect - had + past participle

In the above sentences have/has/had everything is used as auxiliary verbs or helping verbs.
There are sentences with 'have' as the main verb as well. Eg i have the money, i had my dinner.
Now if you want to express these sentences in perfect tenses we use have had or had had

I had had my dinner when you called me out. Here we are speaking about two completed actions and had had is for the first completed action.

Its similar to every past perfect sentences we see with only difference being one had is acting as a helping verb and another as a main verb.

Has had/have had are for present perfect tenses.

'has had' is for third person singular subjects (she, he, it), and 'have had' is for the other persons and numbers (I, you, they). Hope this helps.

We use Past perfect when we want to show a transition from one time frame to another frame
e.g.Before you asked me to stop him, Howard had left for the meeting... here we want to show transition to show act of leaving happened first.

BUT in the question we are stating a fact then why we need Past Perfect, PAST tense should suffice. Consider following sentences, do you think there is any error in these...
a/ Though he was successful in engineering, Howard chose consulting as a career
b/ Though he had success in engineering, Howard chose consulting as a career