Preface:
There seems to be a lot of confusion on what qualifies a thread for being locked/hidden. Generally this is requested when an OP’s help request is resolved, but this is not the appropriate response. The devforums are a public resource, rather than a private discussion between the OP and the solver. Rather than purely consuming from the forum and giving nothing in return, help requests are meant to provide a resource for everyone – not just the OP – that encounters the problem.
What shouldn’t be locked/deleted:
You posted something you regret
Sometimes we make mistakes. I may ask “What address is the RDC conference at?” despite it being included in the email. When this is pointed out, I may be embarrassed by my mistake and want it deleted. Threads like this should not be deleted. If you did not notice the address in the email, more than likely others have missed it as well. You may be more familiar with this concept as it was explained in school: “Don’t be afraid to ask a question, because others probably have it as well.” If the thread is deleted/hidden, anyone else that does have the same question will need to ask it again.
Someone solved your thread
You should not request your thread be locked because someone solved your problem. This is what the mark-as-solution 
button is for – not a thread lock. Users may improve upon the existing answers, provide a .rbxm file as a resource, provide different solutions that help others with similar issues that are reading the thread, etc. Users with the same problem may also return to the thread in the future to ask for clarification, ask if there’s been any change in suggested solutions, etc. Locking the thread prevents all of this from happening.
You solved your own problem without the help of the forum
As mentioned earlier, the forums are not meant to be a resource that someone consumes and never gives anything back to. It would be impolite to ask others to take time out of their day to help you but refuse to do the same. If you ask for help on the forums and end up finding a solution yourself or in another medium, the courteous thing to do would be to post the solution on your thread, rather than request it be closed. Otherwise, others with similar problems in the future will not be able to utilize the thread.
What should be locked/deleted:
The discussion has veered far off-topic from what anyone could find valuable
Ideally posts would be flagged and deleted before a thread gets substantially derailed, but sometimes long chains of bad discussion make it through. When this happens, it would be impractical to get it back on track so that constructive discussion could resume in the future. Locking would be appropriate because the topic has lost all potential to be a constructive resource – resuming with a blank state would be more beneficial.
There is a duplicate topic on the devforums
Note that “devforums” is emphasized. If the question has an answer not on the devforums, refer to “You posted something you regret”. If it’s already on the devforums, we would ideally merge the duplicate into the original – this keeps the topic open for those who face similar problems, but aggregates all discussion into a centralized place that makes it easier to discover.
The topic has no value to begin with
If the topic has no value to begin with (e.g. is toxic like politics, is a troll, etc), there is no need to keep it available as a resource, so it is more than appropriate to lock and/or delete.