The reason why you may want amend your claims pursuant to Art. 19 PCT is to better define your claimed scope for the purpose of provisional protection conferred by the PCT application (in those national states where provisional protection exists, e.g. many European countries. )
These amended claims will be published together with the international application, and they are the ones taken into account for provisional protection. If no claim amendments are filed under Art. 19 PCT, then the original claims of the international application are the ones taken into account for provisional protection.
Usually one files such amendments when she/he is aware of a third party that may potentially fall within a better defined claimed scope. It is quite uncommon to see an international application published with such claim amendments because the independent claims typically have a broad scope and most embodiments disclosed in the application fall therein. However, it may happen that an aspect of the invention was not claimed in the PCT, or the WO-ISA reveals a clarity problem that makes unclear what is actually claimed, then you may want to file the claim amendments to fix the problem.
Bear in mind that the national patent law of each country may set out requirements for the provisional protection to be effective, normally that a translation of the claims into the official language of the country is filed with the national patent office of the country or sent to the third party potentially falling within the claimed scope.
Claim amendments pursuant to Art. 34 PCT may only be filed when International Preliminary Examination is requested so that the claims as amended are examined in Chapter II. This, in turn, may lead to a positive IPRP if the ISR was negative, i.e. you get a positive opinion on the patentability of the claimed subject-matter owing to the claim amendments and, optionally, the remarks in favor of the patentability of the claimed subject-matter you put forward in Chapter II.
As you point out, you may amend the claims when entering into a national or regional phase, so it is not necessary to amend the claims under Art. 19 or Art. 34 PCT.