We split the code so that everytime the field routine is called up we don't have to do a lookup again and again for each record on the database table for select query.
For example if you have 50000 records and you write a lookup(select statement) in the field routine directly then for each record code have to read the data from database again and again for select query. That means 50000 times you are reading the data from the database table.It would consume a lot of time
If you write the code in start routine then only once the select statement gets exceuted and it stores the data in an internal table so the selct query gets only executed once for 50000 records and that would save lot of time and performance would improve. Then we use this internal table in the field routine to populate the values.
Therefore we write the select statement in start routine and then read the data from internal table in field routine.