The issue of "dressing more formally to advance in one's career" got a ton of comments on ask a manager recently. A vocal minority believed that "dressing for success" was outrageously superficial, sexist, and/or not the business of manager to give advice on. But most commenters thought that being overly shlubby/at the absolute minimum required by the dress code could be a hindrance to being taken seriously.
The environmental biz is interesting in this respect because we have some folks who go outside and get grubby, and other folks who may be entirely client-facing and may dress quite formally. So if you're wandering around the office, you're likely to see a wide range of formality, and the grubby people are entirely justified in what they're wearing. Tracking giant clods of dirt everywhere/having small critters crawling in your hair is still frowned upon, though.
So if I'm working in the office all day, I would not look out of place wearing field gear. However, I am not just a field tech. I am a scientist and writer and manager and occasional lecturer. By being somewhat formally dressed when not planning on going into the field, I am projecting the idea that I belong in the office, that I understand the social norms of the business and can be relied upon to be appropriate at high-level meetings.
The same idea holds true in the field. It is not a great idea to show up at a field site with a shiny hard hat, unscuffed boots, a brand-new field bag/clipboard, and pristine field clothes, because everyone will assume that you've never been outside before and it will be that much harder to be taken seriously.
You can overcome being underdressed in the office and overdressed in the field by demonstrating your competence at whatever you're doing. But the fact is that we live in a world of appearances, and it's far easier to start out ahead than to work to prove that you belong there.