Senior officers are the ones giving the orders. No-one gives the Captain (or in reality, any of the more senior grades) punishment detail.
TUCKER: Almost. Got it.
REED: If the Captain learns about this we'll both be scrubbing plasma conduits for a month.
ENT: Dead Stop
and
RANSOM: I've got her emptying {bleep} out of the holodeck's [bleep]
filter!
FREEMAN: ...Ugh. People really use it for that?
RANSOM: Oh, yeah. It's mostly that.
ST:LD Moist Vessel
Being promoted means that your job becomes more difficult (since it often requires you to supervise less senior grades) but also more exciting. Additionally, you're privy to closely guarded information about ship disposition, missions and starfleet's planning.
ROSS: I just got word. Captain Bennet's promotion has come through. At my recommendation, Starfleet's putting her in charge of Seventh Tactical Wing. She's one of the best adjutants I've ever had, strong grasp of strategy and an ability to see the big picture.
DS9: Behind The Lines
and
ENSIGN BOIMLER: We never know anything!
ENSIGN MARINER: We're 'lower decks'. No one ever tells us what's going on. We're not important enough!
ST:LD - Veritas
Lower ranks are given the crappiest jobs and night shift details.
ROM: No, brother. In appreciation of my work the chief has promoted me to the day shift. He says I have a promising future in Engineering.
DS9: The Assignment
You get extra training opportunities not available to lower ranks.
RANSOM: Lieutenant Mariner, report to the officers' lounge for ... management training.
ST:LD - Moist Vessel
You're also given additional respect by your peers, which I would assume is very nice.
SISKO: I remember when I got promoted to lieutenant. It took me a while to get used to being called "Sir" by my friends who were still ensigns.
DS9: Accession
Having the lowest rank means that you need to work much harder to gain respect from more senior grades.
SISKO: Congratulations, Lieutenant. I want you to take a good look around. You have just agreed to take responsibility for the mental health of everyone in this room. You have your work cut out for you.
BASHIR: Well, I'm glad they made you a lieutenant. It would have been hard taking advice from an ensign.
DS9: Afterimage
Conversely, a failure to draw promotion suggests that you're not doing a good job and looks bad on your record, preventing you from getting more interesting assignments in the future.
KIRK: Yes. He had been at the Academy for an unusually long time as an instructor. As a result, he was late in being assigned to a starship. The delay, he felt, looked bad on his record. My action, he believed, made things worse.
TOS: Court Martial
Oh, and being promoted from Ensign to Lieutenant means you get your own room without having to share with a smelly, messy roommate or sleep in a corridor
LAVELLE: Promote me, please, so I can make Lieutenant and have my own room.
TNG: Lower Decks
and
ENSIGN BOIMLER: You get your own quarters?
LIEUTENANT MARINER: Yeah. It's the worst.
ST:LD - Moist Vessel