The questions contains some (in my view) flawed assumptions related to the purpose of the the Opposition in a parliamentary state.
Power without oversight or scrutiny is generally regarded as a Bad Thing, as it allows the people in power to do whatever they like without anyone knowing about it or trying to stop them. Hence the purpose of the Opposition is to provide that scrutiny by forcing the Government to publicly justify everything they do, to draw attention to matters of public interest, and to try and persuade the Government to change their plans where the Opposition disagrees with them.
Now, practices vary between countries, so I'm going to focus on the UK Parliament. While it's tempting to regard passing legislation as the primary role of a legislature, it is merely one of (at least) 3 functions.
The first hour of every sitting in the House of Commons (excluding Fridays) is Question Time, where MPs of all parties ask members of the Government questions relating to their departmental responsibilities. In addition, ministers routinely make statements - and take questions - on important matters; and failing that, MPs can ask the Speaker to force a minister to make a statement on a specific matter if the government haven't already offered to do so.
All of this fulfils the scrutiny requirement described above. It is essential to a functioning democracy to do this - and it requires the Government's biggest critics to be present and not excluded.
Another function of the legislature is to debate specific matters of interest, which the Commons does every week. The subjects can be suggested by MPs, by the Government, or by Opposition parties. These lead to no action, and aren't tied to any legislation; but they are an opportunity for interested MPs to voice their opinion.
And as regards legislation: while the result is mostly (but not always) a foregone conclusion, it again gives MPs an opportunity to share their views and make suggestions. And it is not unheard of for suggestions to result in the Government amending their own bill - though when that happens, it's often not clear where the idea might have originated, so it's hard to see the process.
One should also not underestimate how much discussion between parties and MPs happens in private. It's not at all unusual for Governments and Opposition parties to reach agreement on issues outside the Chamber.
My point is: Governments do actually (sometimes) listen to dissenting views in the Chamber, and so it is in everyone's interest for those view to be heard. Ultimately, having the Opposition present makes for better Government.
To respond to a couple of points made by the OP:
In a comment: "People have elected the government to get shit done, so why impede it?": some of the people elected the government to get things done - but what about all the people who voted for other parties? They're still entitled to representation; and although those other parties may not be in power, they still have an important job to do.
"They only create noise and distraction, waste everyone's time.": far from it; scrutiny is essential - otherwise, why bother having a Parliament at all? Governments understandably don't always like the comments and criticism they get - but in a functioning democracy, everyone recognises the need for a system like this, and the perils of not having it.