Question: Why do we use 'pumunta'(completed-past/Infinitive) whe

Pupunta nako* there are no tenses in Tagalog but rather aspects.

@JoshuaC You don't, "Pumunta na ako" is saying you went/done gone already. This is how I see it.Pumunta na ako sa Spain. = I have gone/went to Spain.
*Pa*punta na ako sa spain = I'm on the way to Spain
*Pu*punta ako sa Spain = I will be going to Spain.

@AMBoy That would more accurately mean only "I already went to Spain". ("na" is "already")You should use the "maka (to be able to)" prefix to express "I have already gone/been to Spain" - Nakapuntá na ako sa Spain".

@JoshuaC No, it would not be correct to use "pumunta" for that purpose.The Filipino Present Tense includes the concept of the English Present Tense but goes further to include the Progressive tenses. The idea of an “ongoing” (progressive) event in Filipino may be used in the past, present, or future through the use of time markers and context. Filipino does not have the “to be” verb which is used for the English progressive tenses. I suppose that one reason why the concept of “Aspect” came about is to disengage native speakers of English from equating English tense with Filipino tense. Filipinos would normally think in terms of “tense” instead of “aspect” though because as far as we are concerned, the progressive tenses and the subjunctive mood are already covered by our Filipino tenses anyway.
The root word “punta” is often used to mean “destination”.Saán ang punta mo? = lit: Where is your destination? = Where are you going?Pumuntá (infinitive/imperative): pumuntá (past), pumúpunta (present), púpunta (future)The meaning of the present tense “pumupunta” is limited to the idea of a regularly or constantly occurring event. It cannot be used to mean an ongoing (present progressive) event. The idea of “aspect”, therefore, becomes unnecessary in this particular case.PUMÚPUNTA siya dito araw-araw. = He/She COMES here everyday.Kailan ka PUMÚPUNTA sa palengke? = When DO you GO to the market?It might help to think of the ongoing concept of “pumupunta” as discrete points in time and space that are linked linearly until “interrupted”. To refer to any point in that line would constitute an interruption and the idea of “ongoing” will no longer apply because the link got cut. That is why if you want to ask someone you meet in the street where he is going, you have to say “Saán ka PÚPUNTA?”. That’s because, at that point, the idea of his “going to his destination” has to start all over again and, therefore, may only be expressed in the future tense. (This explains the answers PinoyTaj and AMBoy correctly gave you.) The response may be, “PÚPUNTA ako sa ....” (lit: I will be going to = I am going to). However, you might also hear “PAPUNTÁ” used instead of “pupunta”. The “pa” prefix gives the meaning of “direction”. Pakanan = towards the right
Pakaliwâ = towards the left
Pasilangan = towards the east
Pailalim = towards the underside (of something); not to be confused with “pailalím”, which means “underhanded(ly)”“Papuntá” is not a verb. It translates to “going towards (a general direction)”. Saan ka PAPUNTA? = lit: Where are you going towards? = Where are you going? (This is more appropriate for asking someone who is about to go somewhere but you have no clue as to where because he has not made any movement to clearly indicate direction.)PAPUNTA ako sa palengke. = lit: I am going towards the market. = I am going to the market. (This explains the answer AMBoy correctly gave you.)