a) Many time out of date statistics or missing indexes can lead to parallelism in OLTP environment. Contrarily to OALP in most OLTP environment a transaction touches few numbers of rows so large table sca n is not desired which generally uses parallel query executions.
b) Say you have highly scalable OLTP database with more than 1000 transactions per second. If a query or stored procedure uses parallelism it can spawn multiple threads per request this in turn increase CPU and amount of threads that can be handled per seconds (1000 threads x 3 = 3000 threads).
In certain situation you can limit max degree of parallelism server configuration to 1 which in turn disables parallelism.
Keep in mind if you set Max Degree of Parallelism to 1, if you use query hint OPTION MAXDOP it would override server level parallelism settings.