Especially at conferences.
If you walk in late, you lose the opportunity to ask a poignant questions by bringing in the risk that the speaker had already addressed the topic before you walked in.
It’s also distracting to the other participants, shows a lack of respect for the speaker and the topic/work they are presenting, and illustrates that you are only attending to get a certificate of completion for continuing education.
On that note, don’t try to ask the nice gentleman handing out the CE slips if he can give you one so you can leave early. That nice gentleman might be me, and you will definitely wish you hadn’t.