Establishing credibility with the listener is the bridge to developing trust in any area of one's life, professional and personal. We all want to be heard, to be visible, to count. For those persons whose English is a second language, the need to be understood clearly may be more difficult. Differences in language rhythms, stress, information and pitch can make speaking and hearing another language more challenging than reading or writing it. Someone with an impressive vocabulary and knowledge of grammar can still experience difficulty sending and receiving communication.
Stress is a factor in communications as well.
If the speaker is afraid that the listener will lose interest, the anxiety will show itself, perhaps by speaking too quickly. Coupling that with pronunciation difficulties can create an anxiety-inducing situation for all involved. They add flavor and create interest. When there's a strain for the speaker who wishes to be understood, then some helpful techniques may be needed. I always encourage language learners to watch the news, listen to the radio and records.