Field formulas are always able to edit the data with which they are working.  With merge report formulas, the data is always read-only.  However, with the other four types of formulas there is a choice, although pre-edit formulas are only given the choice for current entry of the primary form.  Why should a form be writable versus read-only?  Generally, a form should be read-only, unless data will be changing on that form.  This is always the safest, because it prevents accidentally changing data when not intending to.  Also, when a form is not marked writable it does not go through the 8 step edit process.  This is especially important with single entry forms. 

When any form is marked writable, it must be prepared for editing by performing steps 1 and 2 of the edit process.  This seems fairly innocent, especially if there are no pre-edit formulas.  But with single entry forms especially, merely preparing it for edit can, in some cases, cause it to be edited.  If a single entry form has never been filled out and it is marked writable, preparing to edit will actually create the new entry.  Since creation of the new single entry qualifies as a change, the process is guaranteed to proceed past step 3 and be saved, even if the formula using it doesn't make any changes.  Also, if there are pre-save formulas or field formulas that are newer than the data, they will also cause the process to proceed past step 3, even when nothing changed.  If there are field formulas, pre-save or post-save formulas that expect to only be run when data is filled out by the user, it may be suprising when the formula runs seemingly all by itself.  By not making the form writable, all of these issues can be avoided.