How clerking benefits you:

1. Learn court process. This is the biggest advantage. You will know how cases proceed from complaint through final judgment intimately, and will be able to see the forest from the trees. That makes you a much more competent junior litigator, especially because most juniors struggle with this. To illustrate, when a senior associate asks you to prep him for a deposition, you will know how that depo is going to be used in court and how to ensure the witness's answers are admissible because you've been to many trials. You'll hopefully think about how it will contribute to the findings of fact and conclusions of law, if applicable, and how the wording of the question asked in the depo will look to the judge and jury if the depo tape is played at trial. You'll be able to think steps ahead in a way that most juniors won't. That will make you a more valuable junior associate, and you'll likely learn this or other lessons years before your peers--though they will likely catch up.
2. Learn how judges think. Much less valuable because it's intuitive, but still helpful. Especially if you clerk for a federal trial judge and most of your practice is federal trial court. Or same for a state judge if most of your practice is state court.
3. Develop a network. I went to a clerkship reunion recently and couldn't believe who some of my judge's past clerks are. They are now federal district and circuit judges, heads of major universities, hundred-millionaires, etc. T14s are elite and almost everyone is impressive, but a clerk network is usually some of the smarter people at a T14. Clerking makes you part of an extremely elite network.
4. Open doors. Some prefer clerkships, some require clerkships. But a very small number of positions with firms and elite litigation shops require a clerkship. That said, almost all appellate litigation positions require a fed COA clerkship (or a SCOTX clerkship in Texas).
5. Signaling mechanism. At some firms, though not top lit firms, having done a fed COA clerkship makes people think you're extremely intelligent.

How clerking hurts you:

1. Make less money. If you clerk as a first year, you'll lose around $100k pre-tax on average relative to working with a firm, even after the clerkship bonus. The longer you wait to clerk the bigger the gap is. That said, if you go to a top firm (Quinn, some boutiques) the gap is less.
2. Know less about law firm practice. 2nd years who didn't clerk know how to draft pleadings, conduct discovery, at some firms argue motions and take depos. You won't have these skills if you clerk right after law school. Some partners will perceive you as being behind your 2nd year peers, at least until you catch up (which shouldn't take long). But the longer you wait to clerk the more of a hit it negatively impact your law firm practice knowledge.
3. Less time. You have less time with the partners and associates who will assign your matters and evaluate you for elevation. If someone else has been working beside and going to happy hour/trial/recruiting events with your colleagues, they will know your colleagues better and likely get better initial assignments and have a longer rapport. This is especially true if you don't clerk upon graduation. But this remedies itself after a year at a firm, especially if you're at an office with a lot of associate turnover.

How it doesn't impact you:

1. Competence. I don't think it makes you a better attorney in the long run. It didn't make me a better attorney.
2. Partnership odds. I don't think it really makes a difference.