Why did Paul and Silas go into the synagogue when they arrived in Berea?

Paul and Silas entered the Berean synagogue because it was their established custom in those early days. The brethren had sent them by night to Berea, and, coming there, they went into the synagogue of the Jews, just as they always did.

Les explains that Paul and Silas came down from Thessalonica to Berea after being driven out by Jews who would not permit them to preach in the synagogue or anywhere else. When they arrived in Berea, the brethren immediately sent them away by night, and they went into the synagogue of the Jews, as they always did in these early days. The Bereans were more noble than those in Thessalonica because they received the Word with all readiness of mind and searched the Scriptures daily to see whether Paul and Silas’s teaching was true. They only had the Old Testament, and they checked it to see if these things were so. Many of them believed because they searched the Scriptures.

Acts 17:10

And the brethren immediately sent away Paul and Silas by night unto Berea: who coming thither went into the synagogue of the Jews.

Why did Paul and Silas go from Thessalonica to Berea?

Paul and Silas left Thessalonica for Berea because they were driven out by Jews who would not permit them to preach in the synagogue or anywhere else. The brethren immediately sent them away by night to Berea, a few miles down the coast, where they continued preaching in the synagogue.