God has two forms subtle (which is without form) and gross (which has form).
Since Brahman (or God) is undivided, unimaginable, without form, without body etc worshiping it is not possible in that abstractness.
But, God does not give Bhukti (enjoyment) and Mukti (liberation) without upAsana (worsjip) either.That's why forms (that we are acquainted with) are assigned to God.
VinA ChopAsanAm Devi na dadAti phalam nrinAm and also DhyAtah smritah
pujito vA stuto vA namitohapi vA.. (a KulArnava Tantram verse)
Without worship, Devi does not give sAdhaka the fruits. One has to
meditate upon her, remember her, worship her or please her (with
stotrams) or offer prostrations to her..
And, this is (God's murti kalpana) done for our benefits. This is what the Shastras say.
The KulArnava further says
UpAsakAnAm karyArthAm brahmano rupakalpnA.
For the benefits of the upAsaka the formless Brahamn has been assigned
Similarly the NArada PurAna says:
BhaktAnam mokshadAnaya bhavato murtikalpana.
Murtikalpana (assigning definite human-like forms) of God has been
thought of for the purpose of giving moksha to the devotee.
And, the Garura PurAna says:
Amurtyeschat sthiro na swAt tato murtim vichintayet.
If God is without a definite form, his meditation is impossible.
That's why imagine a form of God (murti chintA).
So, the murti kalpanA of God is part of the scriptures only and not our imaginations. How a deity looks, where it is standing/sitting , what is it wearing, what are it's weapons etc are contained in the so-called DhyAna sloka of that respective deity.
In YAmala Tantra, Lord Shiva elaborates this (two-fold Murtis of God) further as follows:
Sthulasukshma vivedena dhyanantu dvividam bhavet | Suksham
mantravapurjyAnam sthulam vigrahachintanam || KarapAdodaRasyAdi rupam
yat sthula vigraham | Sukshamcha prakrite rupam param jyAnamayam
smritam || SukshmadhyAnam maheshAni ! kadAcinna hi jAyate |
SthuladhyAnam maheshAni kritvA mokshmavApnyAt ||
Sthula (gross) and Sukshma (subtle)- DhyAna are of two kinds. Thinking
of the mantra-body of the deity is Sukshama dhyAna and thinking of the
gross body of the deity is Sthula DhyAna. Gross body (Sthula Vigraha)
of the deity is its form that is having limbs, torso, face etc. And, the subtle
form, that is beyond Prakriti (beyond the three gunas that is), jyAnamaya is
the Sukshma Vigraha. Hey MaheshAni, without Sthula dhyAna Sukshma
dhyAna can never be achieved. The spiritual aspirant attains
liberation by Sthula dhyAna.
One more verse from Vishnu YAmala, which explicitly mentions the reason why gross form of the deity is necessary.
MAtastat paramam rupam tanna janati kaschana | KAlyAdyah sthula yad
rupam tadarcchanti divyaukasah ||
[Lord Vishnu says to Adi Shakti] Mother, No one knows your subtle
(Sukshma) and supreme form, That's why the Gods worship your gross form which is
endowed with hands-feet etc.
There are many aspirants who want vision of God. For that they do purascharana of any particular mantra related to that deity. Now, if successful, then God will appear before him/her in his/her gross form i.e with human like features and not in the subtle form which is beyond senses.